My sister (undiagnosed with displayed symptoms of BPD) has very extreme mood swings. I feel like I'm walking on eggshells whenever I'm with my family. I'm personally a big peacekeeper, so things tend to be okay with us one-on-one, but she will go from smiling and joking around to screaming and slamming doors every day, often multiple times a day.
I love her so much, and going no-contact would be impossible since she still lives with our parents and I live at home over school breaks. I know I cannot force her to get help if she doesn't want it, and I know I cannot control her or her emotions.
Does anyone have any suggestions for what I can do to at least relieve some of my own tension at home?
I have a similar issue with my sibling. It was a really long road but eventually my parents started to see that my sibling's behavior was a problem and stopped blaming me for enraging them. My advice would be to really read up on BPD. Knowing why your sister is splitting/having mood swings makes them more predictable for you. It will also help with the eggshell feeling because you will have more context for what will trigger her. If you don't already, start going to therapy with someone who specializes in bpd or family members of bpd people. Your therapist can help you identify specific ways to deal with your sister. What helps me the most is just repeating to myself that my sibling is not reacting rationally. Whatever you did to make her angry probably does not warrant her anger, but don't fight to make it make sense in your head.