For reasons Regulus couldn't parse, the response irritated him- no, infuriated him (which was quite the accomplishment, but if someone was going to do it, it would be Sirius).

He wanted to slam the plate in his hand down or, maybe, throw it at Sirius?

He did neither, because he needed the food. So he just gripped the edges of the plate with tightening fingers.

"Because you're my fucking brother!"