Hi sistoBPD and welcome.

You don't mention if your sister is being treated by a psychiatrist/therapist. Has she been properly dx'ed?

If not, is there any way for you to talk to her about seeking help?

How 'bout yourself? Would talking with a professional about your concerns and fears help you? It might also help you understand her diagnosis and help you set proper boundaries.

Just a few thoughts,

Petunia