Think about the reasons why we allow homosexuality in a moral sense. Because if someone has certain urges and desires to engage with another entity, and that entity consents, and their engaging has no harmful effects to either of them or anyone else in the world, then it should be allowed. This means that if a brother and sister willfully engaged in sexual activities, and it was understood that they would keep it secret (thus avoiding the harm done to family and friends) and would use contraceptives (thus avoiding the genetic harm of incestuous progeny), then by the same principle allowing homosexuality, a brother and sister should be morally allowed to have sex.
Thoughts? And I mean thoughts besides your hot sister