You know, your reasons for not wanting to go are valid and real, and you have the right to look after you. I am dissociative and I assume you are not, but even so I think this still stands: Not only do you have the right to look after you, but you have the right to be looked after by you. Its a subtle difference, but it comes from two different angles, yes?
Anyways... here's what I would do (if I could!): If you want to go visit your mother for Christmas, but at a different time to your brother. Your mother may wish to play happy families for her , but in all honesty isn't happy for anyone and it actually hurts you, so No. Just No. But a compromise if you wish to give one is to visit your mother at a different time.
And, if you don't go at all here's what will happen: the earth will keep on turning, the haters will keep on hating, and others will keep on sticking up for all human beings to have equal rights everywhere.
The End.