Dear Bridget,
Didn't someone once pray, "Dear Lord, make me an instrument of thy peace?" I believe that what you can do, in addition to praying over the situation, is to take the lead and set an example for the rest of your family. You are obviously an adult, so I am assuming you can come and go as you please. If your grandmother is in the area, take the initiative to visit her yourself. If she is not close by, call her.
While "visiting" with her, don't allow yourself to be drawn into the family strife. Tell her that you believe in the old saying, "If you can't say soemthing nice about someone, don't say anything at all.". Tell her about your mother and brother, and try to paint a good picture of them. Tell her that if you don't get together, you are going to miss her around the holidays.
Now, repeat the process with your brother and mother. Reminisce about happier times, when everyone was getting along (There was some time, wasn't there?). Try to act as a go-between with all your family members. Subtly plant the seed that the holidays are for forgiveness and thanks. While Easter is not the holiday we are focusing on, you might want to point out that Jesus broke bread with Judas less than 24 hours before he (Judas) betrayed Him.
In any case, even if your efforts are not successful, you can still set an example by visiting everyone on Thanksgiving and Christmas. In my opinion, many people pay too much attention to "who eats where" instead of enjoying the togetherness. Eat at everyone's (if you're up for it), or eat at noone's. What matters is the fact that you will have shown love and kindness to all your family members.
Hope this helps.
God bless.
I believe that if you maintain a positive attitude toward everyone involved in the "drama", and refuse to be