Welcome Georgia!
You've come to a great place to find other members who will support and validate you, offering encouragement and hope. There is a lot to read here at the site, and reading through other posts can be really helpful too.
Good for you that you've read and educated yourself about BPD, thus helping you to understand some of what is going on with your uBPDm. My mom was like yours, an uBPD. It is a horrendous PD to work with. I'm so thankful that you are supporting your sister!

She needs you very much, especially when the worst of the emotional and verbal attacks are taking place. Holidays can be the worst, and perhaps that is why your mom is making life even harder right now. What do you think?
Keep posting and as you feel free, please share more of your story.
Wools