Yeah, that's his brother! Peter's brother is a perfectly normal human being, because Peter himself would have been a normal human being, had his mother not had an unfortunate run in with some magic while she was pregnant with him.
The two don't properly meet each other until they're both grown and have lived very different lives. Neither really knows what to do with the other. Peter hates being petted on the head, it reminds him of when he was younger and about the size of a large dog and got petted as a way of condescending to him. He lets his brother do it, because he doesn't want to scare him off.