She can't stop him from having a girlfriend. Sometimes there is a clause in the divorce decree that states something to the effect of a partner can't move in, unless married. 

The most she can do is get a court order saying he can't live with or have someone spend the night if they aren't married.  And even that is a stretch, it's usually an agreed upon provision in the order, not something the judge orders.

So, essentially, nothing - except make your lives miserable and alienate the kids. 

She has no say in her ex moving on.  Unless you are a danger to the kids the two of you are fine.  Have your SO tell his ex to stay out of his business.

Unless there's something in the court order about overnights before marriage, there's not a whole lot she can do legally that will succeed.  But she can file all kind of frivolous stuff to cost your BF money and time in court.  And there's a lot she can do outside of legal channels.  She can badmouth you, she can badmouth your boyfriend, she can tell the kids their father has chosen you over them, she can put the kids into a loyalty bind, she can use the kids as weapons to try to destroy your relationship, she can utilize the kids as spies to destroy any semblance of privacy you have.  There's a LOT she can do if she really wants to.   Most BMs can't be bothered to stoop to all that, but there are lots of examples here of vindictive and/or personality disordered BMs who do.

Nope, she can't do shit.  Your SO needs to have a CO in place with clearly defined custody and visitation.  His time with his kids is his time with his kids and he can do on his time what he chooses to do. BM has no say in his time with his kids.

She can try to do whatever she wants legally.  You have the facts on your side.  She needs to know that any crap she pulls will be met with consequences.  Apply those consequences.

If she is reasonable, be reasonable, if she is not reasonable then she has removed the gloves of reasonableness and should not be dealt with reasonably.

Ask yourself, too, is this guy worth it?  Is there some reason this guy w/kids and a toxic BM is more valuable than a man without kids and no BM in the picture?  Better if you find your male equivalent vs. signing up for a lopsided relationship where you'll be expected to, by far, do the majority of the sucking it up and taking it.

who remarried 11 yrs ago the week divorce was finalized while kids were at school (she told them they had a new daddy when they got home) and had been cheating on hubby for several years of their marriage, kidnapped her 2 daughters claiming i was a half naked christian Caucasian prostitute.

Until daughters found out my mum was asian and of their ethnicity and i was born in the same religion as them lol.

but again, bio mum had to do this, it was justified. Eldest sd23.5 told daddy it was his fault mummy turned into a psycho from the trauma of hubby divorcing her.

hmmm lets recap, your mum was a hoebag cheating on your dad, she married her ex high school bf who left his wife to marry your mum and your mum was so traumatized and tried to stick it to daddy that she was so in demand and could hook any man, except she got a male hoebag for a husband while daddy married a younger responsible woman...mmmmm

trust me hun, a shitstorm is on the rise courtesy of bio mum... figure out if you can handle it and if this man really is worth it

She can say and do anything she wants.  If her kids believe her, then what ?    Don't get into a battle with the BM just because you want to win. Or you feel that your BF is the greatest.   You have to remember in the long run SP never win. Kids will always love BM.  And put BM first.  
 

"This situation is nothing like mine, but thanks. ". There are no new situations, just a bunch of old situation that all comes up 
 

OP there is a lot your 'boyfriends' ex can do legally.

She can file motion after motion after motion for anything she wants. If she doesn't like the clothing dh has at home, she can say HE  doesn't provide the child with clothing AT ALL. She can file a motion and report "x' has new job and makes 20k more per year and she wants more cs. Results in dad proving her wrong to a tune of a few grand for appearences in court. Maybe her lawyer fees too.

She can file a motion to be heard because she thinks your mean to her kids...I mean abusing her kids...Then she can turn around and file another motion to keep you away because she report IN HER motion that your driving her kids around drunk.  Then she may start telling your boyfriend you cannot go to school activities because the child is upset. OR your told to stay away at visitation because it upsets the child.

OR she might scheduling the child for every possible activity under the sun, year round. This will keep your boyfriend running around during the week AND all weekends so he wont be home with you.

There are a zillion things she can do IF she is not stable. She can ruin your life and she can also make dad disappear if she wants to.

Your here to either stir up a debate OR you are here because your concerned. There are hundreds of years of experience given to you here for free...Lessons we have learned are everything depends on the mental health of bm and dh too.

Love is not a feeeeeeling OP, it is actions. How old are you op?

Can she legally do anything because she disagrees?

I'm not sure just how much she can do but what she definitely should do is seek some therapy to help cope with the divorce and move on with her life. I'm not certain if she'll ramp up the crazy once she doesn't get what she wants but be prepared in case she does. If she's feeling scorned she will try and pull all sorts of things to make you all miserable.

BM thinks we're damaging the girls by being together and repeatedly tells my partner that he's being selfish and not thinking of his kids.

She doesn't get to dictate who he has around his girls and get to call him selfish. Any talk like that needs to be ignored and only address things that pertain to his daughters and not all that unnecessary back ground talk. Every time she does, he needs to shut it down ASAP.

1. BM did not tell kiddos about divorce until sometime after it was final.

2. She LIVED with your SO UNTIL AFTER divorce was final.

3. She was controlling the entire time that you have been together.

4. She is highly critical of you and SO during the entire time you have been together.

Is this correct? Definitely read posts here to see what other folks have been through with respect to toxic BMs. Mine is affectionately called Toxic Troll. She is a narcissistic personality disordered, sociopathic liar. This does and has "rubbed off" on her progeny.

I am 5 1/2 years into this, we began our relationship as friends 1 1/2 prior to that as friends. He was separated-not-divorced, and in 2016 completed the divorce.

Our BM, Toxic Troll is not as controlling as yours, but she did try to dictate that I could not meet kids until a few months in, and then suddenly it was ok, because shed known a dude for a week and had him staying over and he "whoops!" met the kids.

In my world, the legal "rights" and the BM "birthing rights" are slightly different. Legally, she can TRY to do what she wants, and your SO can spend thousands wrangling with her. Most bio fathers let the bio mother control many aspects of kiddos life. And again, this one sounds VERY controlling, at least until its convenient.

It doesnt sound like she has moved on and she will use the kiddos to control SO.

Just - forewarned is forarmed. Im not advising you to move on, give up, or anything, just be aware of what lies ahead for you and in that way you will be able to make your best decisions for your best interests.

OP, research High Conflict Exes, Pathogenic Parenting, and Parallel Parenting.

The biggest problem with a HC BM is, they seldom stop. She can be a constant disruption in your life indefinitely, even if you never see her. Worst of all, they turn their children (that's right, THEIR, because the kids are her property) against their father and SM. Family Court has an abysmal record of doing anything about Parental Alienation, so imagine signing up for years of psychological warfare. Imagine the long term stress, the physical and mental health issues you'll develop, scrimping and saving because of your SO's legal bills, and the inevitable fighting caused by all of this. Imagine your SO's kids hating you, and not wanting to see their dad anymore. Imagine resentment and depression and anxiety and PTSD. THAT'S what life is like being involved with a man with a HCBM.

OP--I wish you the very best.

GoodLuck