The joke being that they have the same mother and look nearly alike.
Mewsette actually started it, though here it looks as if Giuseppe is the aggressor. He has the bigger ego, though, and seems to be easily set off by signals that I can’t see. I presume Mewsette insulted him.
I also liked the shadows on the floor; it looks more like he’s blessing her.
It took me nearly an hour to scoop the basement litterboxes this morning because they were all being so photogenic.