How would you deal?

Long story TRYING TO make short: my boyfriends baby momma and my boyfriends brother ended up having a baby together in March. They hooked up once and she got pregnant and they’ve been on and off since. Well now they’ve been living together again for about a month in a half. I guess my question would be how would you guys handle this? Day to day, we just ignore them and let them do their thing but I’m just having major anxiety with the holidays coming up. I don’t know how to act around her. Our family is used to doing everything together and this has put a major damper on that. Her and my boyfriend had a terrible relationship before this happened (they didn’t end amicably, they ended with her cheating on him which led to them both having bad drug addictions, a custody battle, etc etc). So it’s not like there was any mutual ground BEFORE they pulled this. My boyfriend and his brother were best friends so he feels majorly betrayed.. it’s just not a good situation all around.

I just hate feeling like the bitch because I don’t want to be around her but she’s no better.. she constantly is telling my mother in law a bunch of crap about me.. and my mother in law just plays along with it.

It’s just all around a toxic situation. I know there’s probably some of you that are going to say it’s none of my/our business and that’s fine but I’m looking for actual advice. I can’t afford a councilor and I’m really at my breaking point with it all.. oh and an added bonus to an already shitty situation.. me and my boyfriend have been dealing with infertility for three years so that as well was a punch in the gut when we found out they get pregnant after getting drunk and hooking up one time 😭. I just hate feeling this way... this hatred I feel for her and the entire situation is unreal and I just want it to stop.. I’ve tried being positive, minding my own business but it still infuriates me everytime I hear her talking about me or every time she’s around. I also think it would be different if when i met my boyfriend she was around, but that was totally not the case.. she wanted nothing to do with his family (so I was used to her just being the kids mom nothing more) and now I have to get used to her being technically a sister in law. I just don’t think I’ll ever get over it. I’ve considered leaving my boyfriend because he doesn’t deserve to deal with me like

This, but I know he doesn’t deserve to get left over my insecurities either... help...