I am really torn about what to do. On the one hand, we can not go and just spend the weekend alone, but it feels sort of pathetic and not like real Christmas. Like, what are we going to do all weekend? We already have done a lot of the usual holiday activities with the kids. Our friends are all traveling or celebrating with their families.
You have to decide if your desire to be entertained outweighs your covid concerns.
Did you only ask the brother & kids to self-test? Because anyone could be carrying it, and the testing requirement should stand for all equally.
Agree with the PP who suggested that you recognize that you guys are both on a spectrum to see it respectfully. If you choose not to go, that's fine--you guys can find plenty to do to entertain yourselves.