I am unsure whether mom gets a bit tipsy just at holiday time, or if this is chronic. Worst case scenario, if mom is into the martinis every night, perhaps a chat with dad about an intervention might be in order. Regardless, watching someone you love get drunk is depressing.
It's important to take care of yourself. You can't change mom; only she can want sobriety for herself. But, you don't have to stand in her path.
Make plans! When dinner is over, and before mom gets too far gone, go visit friends. If that's not possible, find a local Al-Anon or Children of Alcoholics meeting. There has to be one, as holidays are difficult for anyone touched by active alcoholism or drug use.
If you can't leave, be the hero. You can wash the dishes and clean up the kitchen. No one will bother you there. It's also acceptable to excuse yourself after dessert and take a nap. The idea is to be proactive in how you handle the family.
Don't depend on luck or serendipity for the situation to resolve itself. The patterns of behavior have been well established.