I have BPD and lately have had a good amount of control over it. Because of coronavirus, we’re living with my MIL in a rural area because it’s safer than our house in the city. We’ve been here for 6 months. My MIL has mood swings. Everything will be fine and then the smallest percipient slight will trigger what my husband calls “tantrums.” I have stayed quiet and forgiving for months, if not years considering the extent of our relationship. It’s obviously harder now to deal with it living with her. 
Today she came home livid that we did not go to the store with her to pick up her forgotten cell phone. She asked us to, we said no, so she left. When she got back she was passive aggressive and then outright aggressive towards us, making demeaning comments towards my husband. 
After a couple hours I tried to calmly talk with her and tell her that I felt like she was bullying my husband. She immediately got really defensive and started yelling. She then went to my husband (he was in our room) where she proceeded to yell about me. I took a couple deep breaths then joined them. That’s when everything blew tf up. My husband agrees with me that she started the fight and I agree with him that I escalated it. Right now I’m looking for support. I’m going to talk to my therapist about it tomorrow. I can feel myself shutting down towards her. I’m not even that angry anymore I just don’t want her in my life. But obviously we live with her for safety and she’s family so what can I do but try to work it out. My husband defended me when she started yelling about me. He’s a good guy.