> Tempted to have Damian bolt to answer the doorbell as yet another excuse to duck out on all the family gathering awkwardness and avoid arguing with Tim, then despair inwardly when it turns out it's not someone he can just run past to sneak out of the Manor for the night.

What, he doesn't automatically love his new sibling for giving Dick and Babs grief?

You could always just have him linger on the outside of the room with Cassandra, just staring at everyone disapprovingly and not saying anything, because that's what they do.