D.,
This is not a rant. it's just really short and truncated because I have an infant son who is going to wake really soon.
Stop looking for perfection. Look to have 2 hours, maybe 3, where the family is together and agreed not to argue.
Get to Al-Anon. Get as many family members to at least one meeting, preferably 6-8 as close together. Nearly impossible? What would you do for this Christmas?
Hire an outside person to be a referee. No, really. Not kidding. Or invite a family friend who is respected by all sides. Maybe a parish pastor? Someone who will step in to say - hey, this doesn't belong here. Leave it for tomorrow. Today is a memory-making day. Someone to defuse a situation before it gets out of hand. someone neutral.
Also, forestall any efforts at raising voices by having EVERYONE go down to a whisper at the first sign of trouble (a voice rising very slowly is usually it!) Any loudness on one person's part will immediately be noticeable. Whether they can return to themselves to respond appropriately is a different matter.
What you are dealing with is DRY DRUNK. Drunk without the benefit or excuse of alcohol. problem still there, but unmedicated with beverage of choice. And truly, alcohol or drugs become MEDICINE to the person using them to escape PAIN. And she obviously has a lot. it is not her FAULT - but it is viewed that way. it is her pain, and her lack of understanding of how to deal with it appropriately that gets her in trouble. Blame is not to be put on her, but WORK is required on both sides to make life manageable.
Acknowledge her statements briefly, without emotion, and with understanding. One minute at a time you are trying to get through this holiday to make a memory for YOU and YOUR FAMILY, not just your fiancé. One minute at a time SHE is trying to make it through without a drink.
I can't tell you to marry the girl, and her sister can't rule your life.
Stop striving for VERY NICE and settle for some work. Because living with alcoholism in the family IS WORK and it is work for the whole family. The person doesn't choose alcoholism, and you don't choose family. Well, when you get married you do. ; )
Good luck! And I hope some of this resonates with you. But really, get to a meeting. And don't do something silly like try to get to one far away from where you live. Because EVERYONE ELSE DOES THAT FIRST, and they're all anonymous anyway.
M.