Hi PolarPam and welcome! So glad you found us, you will find a lot of support and insight here!
I so relate to being estranged from family because of BPD... .It's soo painful! I've radically accepted the reality of the situation, but I still mourn these losses (of the fantasy family) every day.
It sounds like you may be stuck in the Fear, Obligation, Guilt (FOG) place, and are possibly making decisions that aren't really truly you? Here's a great reading about the FOG that may help... .
https://bpdfamily.com/content/emotional-blackmail-fear-obligation-and-guilt-fogHave you tried to spend time around your brother and nephew? If so, how did your mother react to it?
At some point you may chose to set some boundaries with your mother about your own life and what behaviors you will be around. Here's a good way to start when you are ready... .
https://bpdfamily.com/content/setting-boundariesOn this site you will find many tools on how to better communicate with a pwBPD, and also a lot of people who are struggling with the same issues. I hope you will find what you need here!
*Hugs!*
Charlie