re: My brother in law is wearing a Karl Marx t shirt at easter dinner

Tell him a Graucho Marx shirt would have been way more edgy. Then call him a poser and tell him to pass the kosher lamb.

Eta
The more I think about it, not sure how you didn't say something almost immediately upon seeing him.

"Heyyyy there he is. What's up Clayton. Oh wow, look at that, what a stupid fricking shirt you got on there."