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.