I just found out last night that she has been contacting my to go out to dinner & hang-out. Telling him to tell my father she said hi & that she misses them.
I meant they had a dinner and hang-out. I think what's helpful is to move to the center from either your brother and your ex. You may have a potential
karpman drama triangle. The link will explain drama triangles and how to cope.
If she's threatening you with cops your split black. Dichotomous thinking is common with pwBPD and they view the world as either black or white or all good or all bad.
There may be more tension for you if she adds a third person to shift tension. I'm not saying that your triangulated with your brother. What I am saying is to not take either side and create more tension for yourself.
A pwBPD will shift from all 3 positions of victim, persecutor or savior. As an example it could be possible that she was saying bad things about you with your brother if she doesn't see good qualities in you. This may make her a victim, you persecutor and him as savior. I'm sorry it's difficult with your brother.
I think the best course is to take neither side and don't respond to hostile messages.