Let's look at it another angle. Perhaps Credence is a Dumbledore, but not a sibling. Maybe he's the son of another Dumbledore, specifically the only other one who could have a grown son — Aberforth Dumbledore. This would be the most plausible way to relate Credence and Albus, especially since he did have a phoenix come to him. Maybe Aberforth had an affair with a woman who was originally from America, and that's why she was on the boat going back. Or perhaps she was running away from England because of personal trouble. There are a lot of ways this could play out, if we're determined to believe he is related to the Dumbledores in some way.