It sounds like your sister is having a hard time. If she's taking meds and in treatment then, unfortunately, there is nothing anyone can do to help her get the motivation to be well & function normally.
A beautiful life is possible with Bipolar disorder. Like any other illness or neurological disorder it requires motivation
& excellent care to function normally, but it is possible if one chooses. This whole scenario sounds like a big "F-You."
It's really unusual for someone with Bipolar disorder to be hospitalized so often. Her treatment is clearly not working for her, which may be due to her care providers or maybe due to her "F-You."
As for binge eating & bulimia, most meds for Bipolar disorder cause weight gain. Though no one knows exactly why, there's an idea that the body starts feeding fat instead of muscle, so the muscles scream "FEED ME!!!!" The body overrides the mind & is constantly hungry "to survive." I cannot even begin to communicate how maddening it can be. Insane quantities of food can be consumed and one is still ravenous. It's awful. It also causes metabolic syndrome, an extreme form of diabeties. The meds really mess with metabolism.
That said, Zyprexa (one of the double edged sword drugs) is a miracle. Zyprexa Zydis is an accute version & if one is in crisis or needs help immediately that would be the pill to take. It totally prevents the need for hospitalization (provided she's not seeking hospitalization for her own personal reasons). If you're curious, query her on her meds.
Someone here said something (awesome) that sums it up: Meds suck donkey (kong), but life without meds sucks syphillatic donkey (kong). I'll never forget that! It's so true!
Also, binge eating is a symptom associated with Bipolar II. This all probably contributes to her bulimia, since weight gain & binge eating is such a heavy load to bear.
All you can do is let her go, you know? There's nothing you or anyone can do but keep living life. Remember that the decisions she makes are hers, even if they are horrifying. The divorce rate among Bipolar individuals is so high, as is suicide. Her "F-You" attitude puts her right in those statistics.
Most importantly, you can "be there" for her by being there for her children. They are the ones without choice here. You clearly have a lot on your plate. Focus on your family first, and then her kids after
your other priorities.
Don't feel guilty about taking care of you & yours first. There's only so much you can do without becoming crazy yourself & your family needs you.
If you want to visit her in the hospital, you can help her by making it a brief visit, telling her you love & support her and let her how her kids are doing. If she gets mad, you know she's in a safe place to act out. It's really unfortunate she's doing this to them. It makes me really sad to think about what they're going through. Controlling or no, they are blessed by having a grandmother to care for them. Can you imagine if they didn't? Ugh.Do not give her any more money.
Spending is notorious with Bipolar disorder. Decline respectfully. Your kids need that money far more than she does. If you feel like giving her money, take her kids out & spend money with them instead (or give them spending money, because they probably don't get any from her).
To summarize: her illness is no excuse for her actions.
It never is. There's no reason she should be in the hospital so much or watching tv all day. Success with Bipolar is all in the individual engaging in a treatment plan and living life. One can learn to self-medicate with prescriptions during a crisis (which should NOT be happening if she is properly medicated) & learn coping skills. If she's been in the hospital that much then she knows exactly what she needs to do to be well, she's just refusing to do it.
I can't imagine how this burden is tolling on everyone involved. If you ever need to vent there are a lot of support groups that help families of the mentally ill.
Good luck http://www.nami.org/