Dude disfunctionaly family!
Keep the eggnog scotch free and the shouting will subside.
Or just point out that no matter what they think Thanksgiving and Christmas are about they're WRONG!! It's all about getting the family together and talking about the good times because you don't get many chances to have a whole family together. Kids grow up and move away, marriages mean that you have to arrange to meet two families, people grow old and die. Serve up a big 'ol plate of STFU to them and remember you're supposed to love each other.