I can see how they're different with respect to family names. But biologically speaking, genetic diseases have the same chances whether uncle's son or aunt's son, they are both the girl's first cousins, so they both have the same degree of relativity. So genetically, the aunt's son and the uncle's daughter are the in the same genetic clan as the uncle's son and the uncle's daughter.

I'm not always right, but I'm never wrong -Garfield