Let's say that Credence is really Aurelius and he's Albus's blood brother. Since Albus's mother has been long dead by the time Credence is born (which is probably sometime around 1908), she can't have given birth to him. Which means he would have to come from an affair of his father's, making him a half-brother to the Dumbledore siblings. However, Percival Dumbledore was in Azkaban for attacking the boys who hurt Ariana, so he couldn't have fathered any other sons. I highly doubt the Dementors were letting anyone have conjugal visits in Azkaban and, as we know, Sirius Black as the first person to ever escape the prison. Plus, Percival was also dead before Credence's birth. So unless we do some very creative maths, there's just no possible way that Credence is another Dumbledore sibling.