Hi spine,
Welcome to the BPD Family
Your brother is lucky to have a family that has his back.

You have done a good thing in reaching out to him and letting him know you care and support him. You are definitely not alone in your situation we have other members with daughter-in-laws (DILs), sister-in-laws (SILs), mother-in-laws (MILs) you name it... .we all have someone with BPD/BPD Traits.
Having someone with BPD in your life is tough and unfortunately, your brother's story sounds very familiar to me. The isolating him from others is fairly common. At the heart of BPD is the fear of Abandonment, and folks with BPD tend to be black and white thinkers, so you have a SIL that can't see that your brother can love his sister
and her, she sees it as he loves her
or his sister.
Do you think your brother would be interested in becoming a member here? We have a board for folks in a relationship with someone with BPD that can be really helpful in terms of negotiating issues with his wife. Would your brother consider seeing a Therapist? That could really help him in terms of a place to vent, to get support, to get coping skills in the real world.
The problem with being his sister, is that you can't force either of them to do things they don't want to do. You can't force her to seek therapy and you can't force him to set boundaries with her. We can only control our own actions. We certainly can support you, we can provide information, and tools that you might find helpful when you interact with your SIL and that you can pass on to your brother if he is interested, but we can't make them do what you'd wish they'd do.
When you have a chance you might want to take a look in the box to the right ---> each item is a link to more information.
I'm glad you've joined us there is a lot of information, tools and support to be had here.
Panda39