Etiese in Noah day, he had 3 sons and they had wife's who had children. That is how they repopulated the earth  Noah did not sleep with his daughter-in laws nor there mother with her sons. And remember that they lived for hundreds of years , And.Noah and his wife were still having children in there hundreds and so were there  children . As the children grow up they began to spread over the earth . I'm sure somewhere down the line as they spreaded out over the  earth .there was some inbreeding. But Noah and his family did not have sex with each other to repopulate the earth .

This is an abomination before GOD. How old was the younger sister when it started and how old when she became pregnant . Here in the states if the girl was under the age of 18 the brother would go to jail for statutory rape(adults having sex with minors ) it is a crime . The father would also be jailed for not acting to protect her when she brought it to his attention, and possibly the mother also if she know about it . Even if the girl is now  18 or over and she was a minor when it started ,even if he said she know what she was doing ,he can still be charged . And that is one thing the inmates here don't play . You can do just about anything and be ok with them except harm a child or your parents . You are up a creek with out a paddle . You can kiss your but good bye .