“Siblings on both sides” sounds as though his father and mother may have gone their way living separate lives? So that raises the possibility that he was not raised by his parents, and that his parents might not have known what happened to him?
My guess would be that Joseph's parents weren't married...they never announced either parent's name and I would be hesitant to assume who the actual parents are. Just because there's a man with a similar name doesn't mean he is definitely the father...and even if he is, it doesn't mean the woman he was married to was the mother. This could have been a child born out of wedlock year before he got married.