Barty111,
Welcome. Here's a hug for you.

You are so courageous to take on this challenge. I know it's hard, and my hat goes off to you.
I have a couple questions. It sounds like you're in some type of contact with your sister. How often, and is it required communication because of court issues? I'm wondering if we can help you establish some boundaries around that. As hard as it is to know the place she is in and her demands and expectations, now is the time to focus on you and helping you to get stronger. We can help you with that, step by step.
I want to share a link with you about the children of a BPD.
When the children of a BPD parent are at risk This particular article has really touched my heart because I realize now as an adult how much I needed so desperately the things suggested to help a child of a parent with BPD to survive and make it through life. You have such a wonderful heart to help your nephew, but I very much understand the challenge you face. My mom was an uBPD (undiagnosed BPD). I am still healing, and I am very thankful for this site.
Please keep posting, no matter how small or how big. We are here to care for and help you in this journey.

Wools