Since Cass and Jason have such wildly different kill rules it makes sense that they would dislike each other at least a little bit. However, they are both friendly with Duke, so whenever the three of them are together they both try to be the favorite sibling

Cass and Jason are one of the tougher duos to keep a balance I think Jason is too afraid of Cass to try shit where she can see/know about to the best of his ability and to Cass he's like an especially weird bug she can't puzzle out (like you Died you know what that's like how could you think it's okay to bring to other people)

I don't think the three of em have much time to hang out in costume tbh so they're mostly hanging out in civies or in the Hatch (ie Dukes Turf because he's the only person who legally exists)

the moment Duke leaves for the bathroom or anything it's Dead silent as both Jason and Cass do their best to pretend the other isn't there