What if Credence’s aunt on the ship was actually not his aunt, but his mother?
This would explain why she never shows up, why she didn’t come with Aberforth to claim their son. She’s long dead.
Credence’s mother gets sent away from Godric’s Hollow. At some point after he was born, she and her son go to America for some reason. Now, obviously she’s not going to tell others that she’s the unwed mother of the child she has with her. So she says she’s his aunt, taking him to America because his parents died or something.
Then she drowned trying to save the baby she thought was her son. Aberforth would have received word that both her and their child were dead. He wouldn’t find out that Credence was actually alive for years and years.