Okay, I don't think Sebastian and Maru hate each other. Sebastian and Demetrius do not get along very well, and Sebastian thinks that Maru is attention-seeking... and maybe this is me speaking from a place of having 5 younger siblings whom have all grown up and our relationship evolved, but sibling rivalries when young tend to fade away when adulthood is reached unless there is serious beef. I don't really get that vibe as it isn't mentioned that often, you know?
Now, I'm not saying that some sort of scene where their sibling rivalries are put aside now that they are adults would be a bad thing. I'm not sure it is necessary... some of those rivalries just fade and there's no big reconciliation scene, and sometimes the rivalries don't fade. It would be nice for the rivalry to go away and I get that it is nice to see a big reconciliation scene in as an observer. In my headcanon, though, their rivalry just fades.
