My 50-year old sister is bipolar.

Thanks for your reply, it is helpful.  She had been seeing a psychiatrist, and has been on meds for years.  But I believe that she is tweaking them herself, being unhappy with the weight gain.  My other sister, with whom she is less abusive, is trying to calmly talk to her and get her some help.  She asked about her doc and the meds and got a squirrely answer...so we think the doc is still unaware that she's adjusted them herself.  It's having bad effects on her judgment and behavior, but she seems unaware of that. 

Believe me I know that the meds can make you gain weight-I gained 40 lbs!   And it is hard to get off.  But at least my husband said he'd rather me be a little overweight and no mood swings, than the other way around!  At least she talks to your other sister.  Maybe she should suggest talking to the doc about the weight gain.  I believe there are other psych drugs that can be added to her regimen that will counteract the weight gain/food craving.  Maybe your sister will let your other sister go with her to the doctor so that she can mention what's going on.  I know that the doc will not talk to you about your sister and her file, but I'm not sure if maybe you can call and leave a message for him about what is going on-I think you can do that.  And then he'll bring up that family members are concerned about her and her meds and he can get through to her not to adjust her meds herself, because it could be dangerous.

Yeah, the thing is we don't know her doc's name or anything like that.  Or even the type of bipolar she has.  The "nice" sister, for lack of a better term, lives in another city.  I just moved to the city where the bipolar sister is this year...I had no idea of her mental state.  She fakes it pretty well most of the time. 

Is it possible to get a hold of her med bottles to see who the doctor is on them?  I don't know if she even lets you come into the house, but if so, you could just casually look around or in the med cabinet to see if you can find the bottles.  Or you could mention that a friend needs a recommendation for a psychiatrist and see if you could get the name that way.  I was just trying to think of sneaky ways to get a hold of the doctor's name!  I wish I had more advice for you-I probably would if I had lived with someone with bipolar, but I am the one with it and recognized on my own that I had a problem.

Darn, I had an opportunity to do that.  I think I know where she keeps the meds, and she was out of town.  I was checking her cats so I had access.  But it was after that when this whole thing blew up, so I didn't think of it then.  Thanks, though!

I'm starting to think there is a serious a-hole component.  She's always been difficult, all of our lives, but it is hard to know where her personality stops and the disease begins.  She can be thoughtful and generous and fun.  Right now she is the opposite.

This stemmed from an incident in which she asked me to do a huge favor, and kind of rigged it so "no" wasn't an option.  I went along with it, and of course was angry at myself and at her.  I later talked to her about it, saying I felt she'd treated me badly and whoa -- she had done nothing, and it was my fault that she not only asked a pretty sizable favor and then deliberately never thanked me for it.  And I was a terrible person for even bringing it up.  But we do know she's been messing with her meds, so...?

You might want to read up on borderline personality disorder. I'm not saying your sister may have it, but familiarizing yourself with the illness may give you more insight into her behavior than you will get from finding out more about bipolar.