Playing AC Odyssey over the last two weeks has re-piqued my interest in Greek mythology. If I remember correctly, Cronus married his sister Rhea, while their son Zeus married his sister, Hera. Seeing how both the most powerful of the Olympians as well as their predecessors, the Titans, married within the family, were incestuous relationships considered acceptable in Ancient Greece?
Not to discourage any further answers, but while you wait you might like this older answer by /u/cleopatra_philopater which mentions the acceptability of close-kin marriage in ancient Greece