You do not have to let them in your house. If they show up, you don't even have to answer the door. It might be a good idea to get both an alarm system and cameras put in place now, before she moves to town.

Double check with your lawyer and make sure both your wills and POA's are detailed and up-to-date. Since the cognitive issues could get worse, see if that needs to be directly addressed in some manner. They might try and interfere with your DH's care, but if have the correct paperwork, they will not be able to suceed.

If I were you, I would insist DH see them out of your home, until that becomes impossible. If you feel like they would need to visit your home, do you have a friend or family member who could also be there to run a little interference if necessary?

I'm so sorry you are having to go through this. Your DH is lucky to have you.

You need to get comfortable with calling the cops on them for trespassing. You need to get comfortable standing up to them with legal means if necessary.  You can't play nice anymore. Get video doorbell to record any attempts to enter house.   You can also use the doorbell to communicate without opening the door (and get some good locks on them). It's long overdue to stand up to them but I think it's time.   Do they know about their father's cognitive issues?   Is the paperwork tied up so they can't change it?  Was it writtten in a way that if they challenge they get nothing?    As much as you don't need this drama, you will have to stand your ground with them.  They won't like it and will act up even more I suspect.  I think this is every stepmom nightmare ending.  I'm sorry you have to deal with this.  

Of his estate and make sure he details how estate will be split up.

this ways if he deteriorates further and say sd applies to courts to be executor etc, you can show courts a will to challenge it. Even if sd is unaware her dad made a will appointing you etc, you can still show to court her appointment as executor or power of attorney is invalid. 
 

she won't have authority to make medical decisions or disbursement/management of the estate. Things like property is ensure its in both your names like home, bank accts etc as these do not form part of estate.

with horrible greedy self centred alterior motive sd's like this, you need to pre-empt their dodgy behaviour well in advance so you're protected from their bs.

My DHstb85 is starting to have cognitive issues, too, and I'm dealing with toxic, manipulative, dependent SD60 so I know exactly what you are talking  about.  Yes, this is the stepmom's nightmare.  My SD doesn't know the meaning of boundaries and, like yours, seems to feel this is still her home altho we subsidize her housing elsewhere.

My DD knows the whole situation and insisted we install the Ring cameras outside.  This seems to have had a deterrent effect.  SD used to try to come here in the middle of the night but since DH told her to stop it and since we have the cameras, that has ceased.  We faithfully lock all doors.

Now that DH is slipping mentally a little, I've had to get more frank with her.  She is used to "Daaad" handling her problems since she split with hubby #2 12 years ago.  He has driven her to countless appointments over the years but his driving is getting iffy.  Last month, she was hospitalized for the 3rd time since February (assorted vague symptoms).  She called us to get her and bring her car home since she wasn't supposed to drive.  DH drove her car and she rode with me.  I told her we just couldn't do it anymore.  I was doing 3 things I wasn't supposed to with my eye condition: driving on a rainy night on a highway.  I told her DH was okay on a familiar route but anything could happen if he were driving to unfamiliar places, like that hospital.  She seems to be in denial sometimes.  Other times, she's crying over "Daad's" imminent demise..

I'm paying attention to all the responses you get here.  Probably like you, I smile to myself when I read about the stepmoms counting the days til their SK reach 18.  If only....

My skids aren't rude, at least not to my face lol......but when DH and I built our home he was insistent on having cameras.  The new doorbell cameras are great btw. We began building our home two years into our engagement and initially I thought it was because I had a young teen at the time ( the cameras are a whole other issue in that regard lol) but what I noticed in the years prior was the entitlement of my DHs kids when it came to him and his space. They truly have an attitude of "whats dads is mine" and have/had no problem helping themselves to everything, from the fridge to garage to the sleeping is his bed when we were away one weekend (ewww) we had a brief couple moments where one or the other tried to exert their dominion over our home and DH shut that down pretty quick. He's not perfect but at least he tries and supports me in the peace I seek in my home. We had an instance where his youngest son (34 at the time) was staying with us for a few months and invited his brother and wife over at around 10 pm to enjoy our hot tub with him, (no conversation took place with us about the invite) we saw them arrive on camera, DH met them at the door, told them it was to late an hour and that he had work in the morning and sent them on their way. He knew he needed to shut that down right away to establish the boundary quickly and firmly.

Make sure your paperwork is legally buttoned up.  Get the cameras.. and good locks on your doors.  In your case, I would refuse to allow them in the home..and facilitate perhaps with a third party neutral person for visits with your DH and his daughter outside your home.  

It's great that you've seen a lawyer and already have wills and POAs. Those should protect you if DH declines, but have you considered what happens if you die first. A significant percentage of spouse caregivers die before the spouse. You may want to see an elder care lawyer to review your documents and be sure your spouse is protected from his children (financially) if you die first. 

This is a stepparent's worst nightmare. It's mine too.

My steps aren't rude like yours, but they do assume that they are in charge of their Dad, especially the SD. My DH had a period of mental decline recently (turned out it was mostly a prescription problem and that has resolved) that really scared me. I expected his kids to start their Let's Control Dad dance, but one kid totally ignorned me and the other was in active denial and wanted to know what I was going to do to fix it. Uhm, yeah, sure thing.

DH wants to move near his kids when I retire. To put it bluntly, No Effing Way. Zero. None. When/if we need help with daily activities I'll figure out something then, but in the meantime it's a big fat nope. They can come visit whenever they want to (SS hasn't been to our house in nearly 10 years). I don't think DH can make the trip to see them without me, so we go maybe once a year.

Keep your wills and POAs up to date. I might go so far as to take the documents to a second attorney to make sure there are no loopholes.

My heart truely goes out to you. 

You are not required to open the door for anyone. Much less allow UNWANTED, uninvited family, neighbors OR stangers inside.  I would pay for a lock smith to change all of the locks. 

A will can be challenged if your husband leaves out certain people but it needs to be in context. If there is no explanation why say toxic adult sd's have been left out, a judge can rule in their favour etc if they can prove they are dependent on him financially.

this is why some simply put large assets like house they own in co-ownership with spouse or solely in their name so when spouse dies, other spouse gets it

my elderly father had a will done about 5 yrs ago where i get 60% and my brother 40% of home with reason I cared for him in his old age and when he had to go to hospital, i helped take him etc when my brother cut off contact

my father is now in process of adding me as co-owner of his home because my brothers wife is meddling in my late mothers estate and my dad whilst he is still alive claiming she is entitled to happiness (aka shopping sprees courtesy of dad and mums estate)

now in my case my brother handled my late mums estate in australia illegally against the lw, co-mingled money into he and his wifes bank acct then used the 20-30k as an offset acct to keep their mortgage repayments low which means my inheritance money was helping pay their mortgage and interest yet he refused to pay interest accumulated and made excuses why it took 6 yrs to pay estate money when all he had to do was get money out of a bank acct and pay funeral and tombstone costs.

he behaved so unethically and behind my back without informing me went to claim mums estate in her country of birth which is illegal as under Australian law, he must advise me what he is doing.

my late mothers biggest estate is overseas where my husband is from, i've engaged lawyers to handle the estate 50-50 split as per aussie law and lawyer claimed my brother couldn't claim islamic inheritance law where he gets 2/3rds of estate as a male because that extra money is not for him and his wife and kids to splurge on but as a buffer to take care of me if say my husband died or i was in hardship. Since we're both married, it's unethical for him to take that extra money because he clearly has no intention of keeping that money aside for me so therefore 50-50 split is what should happen.

my dad has seen so many illegal nonsense my brother and his wife both participated in by pocketing mums estate money in their joint bank acct which is illegal under Australian inheritance law and they can be prosecuted if i took this to court so i've used this to my advantage with the estate in malaysia.

my brother was dumb enough for him and his wife email and text my husband a nasty email that i had no shame claiming anything of my mums estate and that i should claim 0 as my husband is well off (my husband has worked at same company 30yrs vs my brother working few yrs) so you can't compare income levels. 
 

my husband accumulated assets and property before marrying me but in my brothers mind, i own half of these but i do not under malaysian law. My brother was stupid enough along with his wife in the nasty email to my husband to state that he cut off contact with my dad over lies he and his wife invented which is why they didn't bother to respond to any messages or emails that dad was seriously ill and in hospital being operated on.

my brother claimed it was because i was co owner of our childhood home that my dad lives in. I was not and my husband told my brother he needed to stop spreading lies and do a title search.

its this sense of entitlement thaty idiot brother and his gold digging wife believing they are good muslims following the law of god blah blah blah when it benefits them financially but they don't really follow things when it doesn't benefit them. They've justified i am entitled to 0% of my mother or dads estate despite me caring for my dad and handling bulk of bills utilities etc before i married my husband when dad was retired. 
 

my dad has never forgotten this and its because of my brother, his gold digger wife and inlaws sense of entitlement that my dad is this yr getting paperwork sorted to add me on as co-owner and i have to pay stamp duty for it so when he dies, my brother cannot touch the home, its mine and cannot be challenged in court

also in my dads will i have been appointed as executor/administrator of his estate and my dad made it clear my brother is not allowed to hold this position with will updated to state due to his conflict of interest and not handling the inheritance and estate settlement in the best interests of the inheritors in my mothers estate, that i am to perform this role. 
 

there comes a point where its smart to foresee the future issues and nip them. So its smart for your husband to add you as co owner or transfer solely into your name. In australia a spouse can be added on free and no stanp duty paid, just transfer fee charges from memory so check what the rules are where you live. At least this addressed the largest chunk of the estate assets.

with regards to you being executor ir administrator of your husbands estate with sole authority to make end of life decisions etc, he needs to provide context why in will so judge will honour this.

if he states clearly that his toxic daughters have not maintained a relationship with him, harassed and threatened abused him throughout his marriage to you and have not behaved in a way which is in your best interests and health, well toxic sd's can sob some fake crocodile tears bullshit sob story in court, judge isn't gonna fall for it.

Sd's can try taking you to court to challenge you being administrator and this is why liquid assets (cash from bank accts need to be in joint names) that ways they cannot use estate money to pay for it

in australia my brother can challenge the estate in court and claim it from dads estate funds (bank acct) but if the only funds remaining are just enough to cover funeral costs and tombstone, and there are no other assets as home is not part of estate, well my brother is responsible for those court costs. Plan ahead to mitigate every dodgy move those toxic people will attempt.

my dad is against his estate being wasted on legal fees. When my brother had done all this illegal stuff on my mothers estate, he even tried to force me to sign a document that i would not take legal action against him which I referred him to aussie estate/inheritance law that stated he had no authority to do that and was in fact illegal. That his job as executor was to pay funeral costs etc and distribute fund to inheritors with a statement breakdown and reciepts and this would be sufficient for courts. My brother knew he had managed things illegally and was trying to cover his arse when it wasn't legal to do that. He was forcing me under duress that i wouldn't get my money unless i signed. Idiot realised he was messing with the wrong person and paid me

my dad therefore is handling his estate in such a way if my brother pursued legal action, hw simply can't because home is not estate and i am legally the owner as i have paid to be a co-owner but he will fake whatever sob story but courts and lawyers can't do anything and i will have the right to recover legal costs off my brother in the case if he is stupid enough to take it to court. 
 

dad has done things in such a way that even oublic trustee, letter of administrator or executor/power of attorney isn't required as liquid cash falls below the amounts requiring this.

i hope this helps you navigate things and others here in us that have dealt with this can offer more advice