I received the following comment from another post, but I'm going to address it today. If any of you have helpful suggestions, I'm sure that "anonymous" will appreciate your comments as well.
"I have a sister (age 45) who has bipolar. I am her 56 year old sister. I am already dreading the holidays. I dread going to see my parents on Thanksgiving and Christmas because I know she will be at their home. I get a knot in the pit of my stomach every time I think about going. She will be there all day on both holidays. She has not spoken to me in 6 months because I talked back to her last February when she called me out of the clear blue sky and swore and screamed at me for 30 minutes for no reason. Any advice would be greatly appreciated."
First of all, let me say that there's no excuse for "bad behavior," whether your sister is bipolar or not. Should an incident like the one you described happen again, i.e. she calls you and screams at you for no reason, I would say, "I'm sorry. This is not acceptable behavior. I'm going to hang up the telephone now. When we can talk together like adults, please call me back."
And then I would hang up. Bipolar mood disorder does not give anyone the right to be abusive. If your sister cannot distinguish between "appropriate and inappropriate" behavior, she should get help from a cognitive therapist.
From reading your comment, it's difficult to provide advice about what you should do about the holidays. I have no idea whether your sister was always unpleasant at the holidays, if her behavior is caused by her medication or because she's not taking any, what your relationship has been over time, what the family dynamics are, whether there are other extended family members present (other than your parents and sister at these holidays) who can act as a buffer, and so much more.
Knowing none of this, let me say this. I don't believe that holidays are the time to resolve family problems; for the most part I believe they aggravate problems. If you and your sister can talk before the holiday and resolve things, that would be great (although from what you've said, it's seems unlikely). If you can't talk with your sister but if you can discuss her behavior with your parents, and seek advice from them before the holidays, that would also be helpful. If none of this is possible, do you have a clergy member who knows your family with whom you could discuss the problems?
If none of these ideas work, perhaps my readers will have advice for you, or you can always contact NAMI and the Depression and Bipolar Support Alliance to see if someone there has advice, or perhaps Dr. Deb Serani, or John McManamy can help.
To see why your sister may have difficulties during the holidays, I googled "holidays with bipolars siblings" and got an interesting hit on WebMD, which might give you her perspective.
Best of luck!