it not sin , it just incest ! an incest sin would be a father sleeping with his son's wife or the wife sleep with her husband's son ect . sister and brother sleeping together is a sin became usually the elder son or daughter in the family suppose to be the parent figure or caretaker for their younger sibling , Patta slept with his step mom palin who is his father 's wife that incest sin and little Aff thought Patta slept with her mohter and for her to share the same lover as her mohter that is an incest sin . basically if you slept with someone that you know will effect them and the people around them mentally you had created a sin on yourself because you are hurting people emotional , buddha teaching said to respect those that raise you and give you life if you bring torment to the one that give you life .you are sinned