I would look up the Myers-Briggs personality classification system and try to check out a book at the library based on this (you can also look up Enneagram). Your mother sounds like a very unhealthy Two. This might help you to understand her better and it might give you ideas on how to respond to her.
In case you don't have time to get to the library be/ now and then e-mail me if you like and I can try summarize some stuff for ya.
I want you to know you are not alone. I want you to know the way your mom behaves is wrong. I agree with Bev...confrontation will likely not be helpful. (I know it isn't in my fam!)
The only thing you can do, imo (and from what my therapist has told me when dealing with my fam) is that we have to be firm and set limits. SOOO hard!
Ginger gave you a great idea...talking more to your sister about it. That sounds excellent!
*hugs* *hugs* *hugs*
I'm sorry you have to deal with this...esp. on the holidays!! keep us posted...and my e-box is always open *more hugs*