Turned out his aunt was his actual birth mother, who had kids with his father too before he was born, but who had passed away in a house fire. The mother who raised him was completely sterile and, in fact, had a hysterectomy at a very young age. He was in complete shock, but said it made sense how close everyone was in the house, despite keeping all the adult stuff behind closed doors.
"I grew up in middle-class suburbia in the Midwest. Everything that is mentioned in the stereotypical free-range childhood is literally all the things my siblings and I experienced growing up. Outside all day, playing with all the other kids in the neighborhood, looking for our friends' bikes on the lawn to find out which house everyone was at."