She gives him a blowjob and her brother already had to fuck her
Sometimes life gives you options so you can choose the one that seems most comfortable to you. That only happens with girls, men have to do what nature forces them to do and they have no other options. So that’s why after this, this kid only has one possible reaction. His sister gives him a cock suck and he, being her brother, then had to fuck her. Because that’s what a male does with any bitch in his family. He can’t let her have a hot pussy and run the risk of any pervert showing up to please her.