I'm only going to respond in this one thread to help have a more consistent message. If you believe there is a pressing question/issue in another thread that's not being addressed, please ask again in this thread.
Some links that can be helpful.
https://bpdfamily.com/message_board/index.php?topic=121673.0https://bpdfamily.com/content/karpman-drama-triangleI think it's wise on your part to post these issues here to get help thinking through how you "handle" your sister.
Manytimes people that aren't "close" to a situation can "see" things that it's hard to see when you are in the middle of things.
At first blush, I don't think that you and your brother are triangulating. Again, you are right to ask and do need to be vigilant.
What I see is you and your brother attempting to form a common front (consistency) to disordered behavior from other family members. If you start talking to your brother and your brother passes along that info to your sister or vice versa, then I see
Does that make sense to you?
I would advise you NOT to share suspicions about "borderline" with your brother. I WOULD encourage you to be explicit about behaviors and speech patterns that are concerning. Perhaps nudge him to explore on his own for what it means.
I would also advise you NOT to apologize unless we have talked it through thoroughly here.
Apologizing to "feel better" rarely works out well. Apologizing when you have done something to apologize for usually does work out well.
I would want to know details on what you would want to apologize for?
So sorry you are finding yourself in this position. We get it and we can help!
Best,
FF