Edit: I was a misinformed and really sleepy, please just disregard this whole thing.

My older sister (let's call her Joy) has BPD, and we've known about it for a while. She's pretty much a stereotypical textbook case (people are all bad one minute, all good the next. Lies, manipulates, etc.), and it's been that way for at least the better part of the decade. I haven't actually lived with her since we were in middle school. She spent most of her teen years bouncing around group homes. She's 19 or so now, and I think she's doing better, but she only started popping by on a regular basis in the last month.

Joy's been through a dozen guys in the past six months (she says it's because they turn abusive), and frequently "borrows" small bills. She drives with just a permit and constantly has some kind fine to pay. She gets a lot of stuff from us, but she just ends up leaving it behind when she moves.

My grandmother (basically my mom, dad died a few years ago) is kind of fed up with it. She's a nice person, and she cares for my sister, but she has no more patience. She says it's because she already used all of the patience she had for Joy. She's tired of giving tenth chances, but I constantly feel like she's being insensitive or unsupportive. She constantly looks at what my sister is getting into with disdain. "She'll break up with him soon" or "She uses that money to buy cigarettes, and I can't keep paying for her addiction" It doesn't help that she already has to deal with the stress of a paranoid divorced sister and practically running the local migrant education program (by local, meaning much of three counties).

Every time I'm with Joy, i'm on her side. Every time I'm with my grandma, I'm on the other side. The thing is, I'm not sure either side is right. I feel like my sister is constantly manipulating me and exploiting my kindness, and I feel like my grandmother is constantly trying to turn me against her. At the same time, I can see that my sister genuinely cares about me and wants to have a good relationship with me, and that my grandmother is trying to be as compassionate as she can. Meanwhile, I'm stuck in the middle, too empathetic to turn away.

I've done some research into BPD, but I never really learned how to deal with her when we were growing up, and I can't find much on how to help. I really just want to learn how to help her, and handle her. She has enough to deal with.