Your Partner And Your Sibling With Borderline Personality Disorder
She may confide in him
It’s common for someone with BPD to confide things in a newcomer, and ask him not to tell anyone, as a form of creating some bond. It’s a bit manipulative, but it’s part of the disorder. So your sibling may tell your partner that, for example, she’s started smoking again, has stopped going to therapy, or has started sleeping with a problematic ex again. And then she’ll ask him not to tell anybody. It will flatter him at first, because he’ll feel like she trusts him.