New thought: if P for some reason ever had a brother he would also be a P.
I might be misremember but I feel like I recall P mentioning he had a brother and I'm sure I'm not confusing that with him making Storm his brother as a cover story. When I first read the section where P visit's his grandpa's home, I was wonder why there was no mention of his brother in the photos but, again, I could have just been misremembering. It is likely however, that P might have half siblings if bloodlines and heirs among peacocks are a thing (I bet there's likely some resentment towards P by his father for that as well) from both his parents.
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.
It's not necessary that P's mom couldn't have another child, it could have also been an issue with her new husband shooting blanks. Though I think you're right about a possible half brother being on his father side since P's talk about his mother doesn't make it seem like she remarried. Though doesn't mean she didn't have another child out of wetlock or just because she's hitting up P for money she wouldn't be hitting up another possible son of hers for money as well.
As well as that for a brother to be in P's life, it would likely have to be from his father's side. And that definitely wouldn't be a cousin from his grandma since she wanted nothing to do with his family any more though a cousin could be as close as siblings if the hinterlands of Brazil anything like the hinterlands of Mexico. As for that drawing, I didn't occur to me that it could be a brother. I thought it might have been either him and his father or grandpa. Or it could have been his father and grandpa. Though could that childish drawing been based on something that actually happened rather than a child's imagination?
I really want to know more about the legends of these Tapir and Goat people.