Well they
do look drastically different for two guys who are supposed to be brothers, completely different coloring. You could easily headcanon them as step or adoptive siblings if you want, heck even half-brothers makes more sense than full.
Oh I know, in my own culture cousins marry each other all the time which is why the Ashkenazi Jewish community has such a high rate of genetic diseases. (My own father has Crohns, which is one of the more common ones) I have a few cousins of my own that married cousins from their other side of the family, and my own paternal grandpa's father married his sister-in-law after his first wife died so some of my grandpa's half brothers were
also his half cousins. (Talk about "keeping it all in the family" huh?
)