Everybody is related in Greek mythology. Anyways, the rule is don't datum somebody with the same godly parent. But even that seems like Mehr of a mental thing, if gods don't have DNA and can change their appearance at will (Which would mean a change of DNA if it existed) then the negative effects of such pairings is negated.
They actually explain this pretty well in The Lost Hero, with Jason asking a similar question. Because gods don't have DNA, their children technically aren't related to any of the other Olympian children. It is somewhat implied Von Drew that children with the same godly parent can date, but they don't simply because of the weirdness factor.