You're doing better than we are.  be prepared for one step forward two steps back, this might not last. 

SD12 is becoming more of a dismissive brat by the day. BM and her family stays up until midnight Christmas Eve to open gifts and my DH tried to talk to SD several times on Christmas Eve and day with no answer or reply. SD sent him a thank you and merry Christmas text at 8 pm on Christmas day.  

It's very likely they don't hate you but are struggling to balance their loyalties.  Knowing Bm is in the next room over may have them concerned about being overheard, even if BM hadn't said something against you (which idk about your situation, in mine I KNEW that was false. lol) they may feel confused and worried about hurting BM's feelings if they show they like you.  In situations like that you could be their favorite person in the world and they still wouldn't ever show it.

So basically, don't worry about that and focus on you and your bf's relationship

I'm glad you are feeling a bit better. I agree with PAI, the kids likely don't actually hate you, they just don't have an option to like you. Read up on parental alienation. Also understand that if these kids have to live with BM most of the time, their priority will be to please her. They don't have the option to get out of whatever game she is playing, so for their own sanity, they're likely going to play the game, too. Basically, they're stuck in an abusive relationship, and they'll do whatever they can for peace.

As for your BF, he's an idiot. He can't fathom that BM would use his kids against him, so he's only focusing on the kids. That's like treating the rash caused by syphilis - the symptoms may improve slightly, but the infection is still there causing more and more problems. He may not want BM back, and he may not want to fight her, but that's precisely what he needs to do.

He NEEDS to show up to collect his kids for the weekends he has custody. When they aren't released to him, he NEEDS to call the police and either have them release the kids to him or file a police report (he should talk to the police beforehand about this plan and explain everything; he'll get better treatment, likely). He NEEDS to file contempt charges every time she doesn't release them to him. He NEEDS to not accept any gifts that comes from BM and send them back, even if it means just putting it back on her driveway. He NEEDS to tell his MIL to butt out of his affairs, and if she chooses to continue with BM, then he needs to cut off his relationship with his mother and let BM facilitate that.

Placating BM doesn't change anything. He IS being controlled. The fact that he doesn't get his kids in his own home on his own time IS control. What he wants isn't what is best for his kids. What he wants is what is easiest and most peaceful. That's not always what is best, especially if BM is alienating the kids against anyone in their lives who genuinely cares for them for no reason other than they exist outside her sphere of control.

He needs to realize his kids may be in a home that outwardly looks peaceful, but they're being hurt emotionally. His choices should be to either fight like hell or stop playing the game. Those two choices help his kids. Playing along with BM only teaches them that how he is treated is okay and how BM is acting is acceptable. It's no different than a parent remaining in an abusive relationship and letting their kids see what happens and learn that it's acceptable.