Home life about to blow up, mother going back to abusive ex

what the title says. im an adult woman (23) living at home until I go to grad school this fall. i have an older sister who lives far away.

TLDR my mom had an extremely emotionally abusive ex boyfriend of ten years. they broke up 10+ times and always got back together within weeks, but when i was in college they separated for good. my mom is trying to be secretive but has been visiting him every week and saying i love you to him over the phone. she’s also made comments about selling our house (presumably to go live with him) and talks about him all the time.

my mom forgives him for everything he did (im not gonna get into it, but Bad Things), but my sister and i had to witness it all as kids and it really fucked us up. neither of us want him in our lives, but our mom keeps forcing him into situations with us without our knowledge to try to push us together.

my sister is furious with her and i am sick to my stomach all the time, but we both don’t want to lose her and don’t want to get into a crazy fight. we’re both really conflict averse from the rough childhood.
it feels like everything is about to blow up but i just have to act normal. my sister isn’t talking to my mom and i have to pretend not to know why.

I don’t know how to proceed. i am thinking about possible scenarios of how it’s going to go down, how im going to lose my mom forever, how my life is going to be ruined because I’ll lose my family and my home.

i tried crisis lines but they were no help. i don’t know how to proceed so im just ruminating all day every day hardly able to work.

wondering if anyone else has suffered with constant rumination. i have ocd and im waiting on therapy, but it could take weeks to months to get off the waiting list.

thanks so much and have a great weekend everyone