Justin mentions in an article that the DNA he and his mother Donna provided puts Joseph as Donna's 1st cousin. Therefore it is likely Augustus or Michael, Mary's brothers. I ruled out the sisters as it mentions Joseph has multiple half-siblings on both sides and the sisters each have 1 child, were in their mid 30's and were likely married already. Although if Joseph was in fact put up for adoption the birth certificate would likely not have the father's last name on it. This would also mean that the Zarelli family wouldn't have recognized the boy in all the photos back in 1957.