There's a moment where P mentions "him" and it's not specified, but he directly mentions photos of his grandfather, father, and himself. That plus the drawing of two peacocks with the goats means there's likely another peacock he refuses to talk about out that's likely a brother.
I wouldn't be surprised about heirs or half-siblings, though. His mother married remarried a rich peacock that eventually divorced her because she couldn't have another child, but he specifically says that he was the only child his mother had. Whether that is because he doesn't want to talk about a brother because of something that happened or it's a half-brother is unclear, but something is going on. Considering his grandmother also remarried but didn't get divorced, she may have had another child and so it could be a cousin, as well. That he's not welcome with her could point to a rift because something happened. Either way, the pictures of the other Peacock stopped at a young age and that's all we know for sure.