Your vote on this answer has already been received
I said lover/spouse, not relative. You can't choose your family, but you can choose who shack up with. People with BPD especially if severe can be physically abusive, and I would NEVER advise any woman to stay with a man who is beating her.
Only God can heal/save. No person/lover/friend can "save" anyone. In some cases if the symptoms are severe, it is best to leave before YOU have a mental breakdown. Dealing with the up and down of BPD is no joke.