It’s time you set up some boundaries with your mother. It’s going to suck, but you need to do it now instead of after when the baby is born.
You need a hard line with her and let her know that you’re not going to let her trash your in-laws anymore.
“Mom, I don’t have it in me to listen to you bad-mouth my in-laws.” And if (realistically, when) she pushes that boundary, you stop talking to her. Yes, she’s going to feel bad. She’s going to be angry. You have to stick to it.
Every time she does something that triggers your anxiety, you exit the conversation.
She won’t like it, but you are not responsible for her emotions. You need to be taking care of you and eventually taking care of that baby. You cannot manage her emotions for her and you’re only going to hurt yourself and your family in the long run by trying.
If she’s not at the baby shower, that’s on her. “She couldn’t make it” is all you have to say to anyone who asks about it.
BPD is tough, but trying to manage the emotions of someone who has BPD is a losing game.