Hi Wintersun!

I think you are very smart to stay away from Sister A and your mom. I still have nightmares of my mom waking me up and either questioning me or raging at me (and did you used to have midnight cleanings where you were woken up to clean your room... .What the heck?)
Anyhoo, stick to your guns and stay with Sister B. No explanation needed, however, you can consider the following for anyone who asks why (not necessarily your mom tho):
1) I do not get to spend much alone time with Sister B when I visit so I decided this time would be a perfect opportunity to do so.
2) I think Sister A has enough on her plate with one overnight guest and I also would like to spend time with Sister B.
3) My mother is a raging fruit loop but you never see it because she hides it well from others. I refuse to subject myself to her abuse.
4) Things are a little tense right now what with the divorce and all so I think my mother needs some alone time to help her find her happy place. I don't know where that place is, but it is not near me.
Okay, so # 3 and #4 are probably not what you had in mind, but what do you think about the first two options? Anyone who asks can also be told you prefer not to discuss it, but that will most likely fuel gossip and get people poking around in stuff they should not be poking at! I can imagine someone saying "but what about how your mother feels"? My reply would be to smile and say "I don't know, why don't you ask her" or "Oh she is fine." Done deal.
As for your mother, well, that is a whole different thing. Whatever you do, try not to be alone with her. You said you fear for your safety with this latest ramp up in rage. Be careful and at the first sign of her raging at you or starting up with ___, walk away. Will you have your own transport? keep your keys on you. keep money and credit cards in your pocket and just walk away. Do not engage. Do not reason. Do not defend or justify. Protecting yourself has to be your priority. You are worth it.
Now, after all that protect yourself talk (I tend to get very passionte! ) if your mom says anything about where you are staying, how would you feel about telling her something like: I do not trust you to not go off on me like you did the last time I visited. I did not deserve it and it hurts and upsets me when you do _____ . I understand you are upset with dad but I am your daughter and I deserve to be spoken to with respect and consideration. I refuse to put myself in that position again.
I have no idea if being that direct with your mom is a good thing. You know yourself best and you know her. In general, I think we (as in all kids of) need to take a stand against the BS that we have always dealt with. But that does not translate to all situations. My mom used to scare the hell out of me and so it was the most empowering thing in the world to speak up and tell her no, ___ off (yeah, I really did), or whatever. The goal is not to get through to her or reason with her. It is simply to say stop treating me like this. Stop talking to me like this. Stop doing ___ I do not deserve it and I will no longer tolerate it.
The behaviors may continue, they may even increase or escalate (in which case you stay far away) but I really think we must speak up and take care of us. We can spend the rest of our lives (or theirs really) trying to make things peaceful or trying to avoid conflict... .but how realistic is that? There are some here who can do that, and I applaud them because it takes a special kind of patience and dedication that I just do not have. My mom and my situation called for very different actions. I think the hardest thing for me to cope with was knowing that my saying no and standing up for myself, moving away, etc, made life with her for my brother and father that much more difficult. It was very hard knowing that, but it was also hard knowing that the two of them spent years watching me be degraded and abused and I could sense their relief that she was focused on me rather than them.
We have a right to stand up for ourselves when we are being abused or treated like we do not matter. We have a right to take care of ourselves. Standing up for ourselves, saying no to the BPD parent and taking care of us in no way changes or limits our love and compassion for them. What it does do is allow us to demonstrate love and compassion for ourselves.
So Wintersun, what do you think? Lets hash out some possible scenarios and things you can say. You will be all right ya know.