I hope he never actually gets to work in politics; the world doesn't need (more) people like this in positions of power!

I'd say, your mom needs to put her foot down, and not just for Christmas, but if she doesn't, well, you can't force her, even though it'd be for her own good, without putting even more stress on her. And the last thing you want is a family-internal power struggle over holiday dinner (believe me, I've been there.

So, instead I'd say, flip your brother the bird: have dinner elsewhere -- possibly don't even tell him where. And proceed to have a wonderful time while he stews in his Pyrrhic victory, alone at your mom's place. Because if he insists on behaving like a toddler, he needs to be treated like one. Ignore the kid and the tantrum will stop after a while, too.
Also, that way you not only spare your mom the stress, but also spare yourself and everyone else a Christmas dinner sitting on the metaphorical volcano. Extra benefit: you come out smelling like roses because you didn't stoop to his level and potential ruination that takes place despite your mom's efforts at least can't be blamed on you, because you didn't "provoke" him (as he'd indubitably turn it around) in any way.