Her threat was nasty, but ridiculous in nature. No law enforcement agency would take it seriously. She doesn't have a mechanical killer whale. I realize the implication is that she might want you out of the picture, but I wouldn't take it seriously without additional evidence.

As long as your husband stands by what he said and keeps her out of your home, the only thing else you can do is maybe put cameras and a security system on your house.

Going forward, if she sends any other vaguely threatening messages, take note and talk to the police about a restraining order.

Being  Bipolar changes things.  She must stick to her protocol, take her meds , stay in medical care and proved she is taking care of herself.  She must also acknowledge her bad behaviors and authentically apologize.  It's not to be used as an excuse. 
 

If you don't know much about Bipolar, look up Julie Fast.  She writes books and has FB groups,  Instagram and a great website.  
 

As for the change in attitude, marriage has a way of blowing the fantasy of parents reuniting out of the water and if it's not acknowledged then,  it festers like it has in your case.  Do not let your guard down, document what you have and file it away.  It's good DH put a boundary in place but he used YOU as the reason , not HER behavior, thereby throwing you under the bus.  In her mind that sets up a scenario that YOU are still the cause of all her troubles with her family.   
 

I think learning more about Bipolar will help you figure out what is related to bipolar and what is character.  Julie has a way of doing that.  

SD in Bipolar, what do you expect.  She basically is not in her right mind.  Whatever you do is not going to effect SD.  SD is in her own little world.  But it is DH kid. He still has responsibilities to his DD.

If you don't want SD in your home, you are perfectly right to feel that way. If you don't want to do, help,SD that up to you. BUT remember she is DH kid and is not going to disappear. He must try to get her in some place where she can live.  You don't want her living on the streets Turing tricks to live.   This is a tough road to go down. What has little happiness in it. I lived it.

I understood how screw up my SD was.  She just could not help herself.  We tryed to get her on disability getting her a place to live ect.  she was a person who needed help.  Thank goodness she actually wasn't mind. I didn't have the guilt that I was part of the craziness a

You must remember DH has guilt  it's his genes in SD.  This came from. Somewhere in the family. There has to be Bipolar person somewhere.  May not be as bad but it's has to be there. Uncle Joe who lives by himself in the woods, or aunt Mary the struggling artist, living by herself painting paintings that no one buys. Or cousin Joe in jail 

hauled off in the funny jacket for as long as she can be held for Psych observations.

Lather... rinse.... repeat.

When she threatens you, file terroristic threat charges. 

Keep up up until she either stops or the authorities actually do something about her threats.

Kee it simple. Make it as painful as possible on her.

Eventually she will no longer be a minor and the threats very well may return her some escalating legal consequences.

My husband many times said its pointless stirring the pot when you are dealing with psychos and narcs. You're at the brunt of their abuse and you can't control how they behave. 

yes you can go to police/court etc for restraining orders, lodge police reports etc for a paper trail, sue them in court etc but they want the drama and you're attention retaliating all their endeavours to cause issues in your marriage and relationship so for many men who have dealt with this, setting a boundary up to protect their current spouse/kids etc from exwife and toxic feral kids of prior marriage and relationships and ignore any talk or abuse is what many tend to do or as i say "zone out".

my husband only found the courage about 1.5 yrs after sd's reinitiated contact after cutting off contact and disappearing for 5.5 yrs to tell them off for their uncalled for and disrespectful behaviour. He reminded them of the hierarchy and who had authority regarding our households and then 2 toddlers and stated very clearly it was not them.

he also reminded eldest sd that biomum and affair stepdad was not of either of our care or concern as they were not our family or part of our inner circle. Sd's and biomum etc have spent the past decade trying to make themselves relevant, my husband ignored them and initially when we'd have outings with his kids from exwife, literally seconds into entering our car it was non stop biomum this snd stepdad that. Any conversation we had was also bio mum this and stepdad that etc. and this was met with silence. No response or reaction, hubby zoned out and refused to give a commentary. You'd think in the 18-27 plus years that sd 18 & sd27 have known their dad, that they'd recognise when daddy is intentionally ignoring anything biomum stepdad related but they're so blazee about it. I recognised it the moment i saw hubby doing that and i'd only known him maybe 5yrs at that point. 
 

it really sums up the negative toxic relationship they have but lucky my husband acknowledged that we have to maintain 2 separate households because of his daughters and toxic exwife and meddling interfering stepdad who feels he has a position of authority to dictate the relationship sd's have with us despite not even living in the same home. 

Now that she is an adult, he addressed it. That is all he can really do.

Bi-polar does make things more complicated, Hopefully she get's on meds that actually work for her and she can live her life and live it well. At the end of the day, no matter how horrible these Skids are to us- we want them to succeed (because then they don't come back with hands out expecting daddy to finance their lives!). 

Her "threat" was ridiculous and nothing serious. If she sent you something about checking your breaklines or something like that- I would be seriously concerned but a killer whale video? That was just looking for a reaction. 

That glitzgirl should have to deal with this.  BUT her SO,  that his DD who needs help.  He must try to get her help. Not try to not deal with her.

I would agree that this is your SD's way of threatening you. While it may seem silly, its real to her. I have teens with bipolar and it can make you paranoid, obsessive, and it can amplify your feelings. If she is refusing treatment, I would do exactly what you are doing, and cut off contact with her. Don't allow her in your home until she gets some help.

My oldest SD wanted me gone, and I have banned her from my home and my life. She still sees her dad, outside of the home. She has bipolar too and has, on occasion, threatened me. I am more afraid that if she were allowed in my home, that she would find a way to falsly accuse me of something - just to ruin my life.

Thankfully, your DH supports you. I would wait for her to get her life in order before you reach out again - if you want to reach out at all. 

I have SD24 Feral Forger who is blocked on my phone and social media.

SD17 Powersulk whose true self came out and threatened me. 48 weeks until she has aged out. Husband doesnt want to rock any boats. Hes in a position where if he doesnt ask questions, he doesnt choose sides, then everything works for him. Im in a position of having someone who has threatened me living in a home I pay for. At 18 all bets are off. SD and your husband can have their relationship outside the home and away from you. Your husband did what mine is doing - shes "sick" so she has that as her crutch and "excuse" for her bad behavior. He addressed it once she aged out. His responsibility to her is whatever relationship they have outside the home.

I told Sd17 PS, "have your visitation time with your father." Im staying way far away from her. I keep busy. I decline giving her rides. She is not invited to my bday dinner. She will be taken off my life insurance as a beneficiary. Im thinking of putting our assets in a trust. I really just want to move on and somewhere else. But shes still a "minor". 42 more weeks.