OP is either trying to adopt a younger sibling to get them out of a bad situation which is commendable, or he is worried that a genetic test might prove he fathered a child with a younger sibling during an incestuous relationship, in which case, the kid would have > 50% of the DNA of both mother and father even if the father is not tested.
Someone will look at that and go, "wow, that's weird, why does this test show (5400 centimorgans) in common with the mother, a mother or father should only be contributing ~3200-3300 cM each. Children that are the byproduct of incest will show greater than than normal similarity with their mother.
The good news for you, OP, is that this by itself does not prove you're responsible, but I would keep my head down if I were you.
