Welcome to the family! We're glad you're here!

This is a great, safe place to bounce around thoughts and openly vent about struggles. We get it. I am no stranger to BPD in-laws...I have a BPD MIL, and the BPD dysfunction is traceable throughout the entire family.
Kudos to you for having compassion for your SIL. It can be tough to tap that empathy when you're hurting, but empathy can and will keep that door open way longer than ultimatums and other unhealthy responses.

I imagine you miss your brother and that you're grieving the loss of a good family dynamic. What's the hardest part about all of this?