They catch her showing her ass for social networks
That his stepfather told him that he had to make sure that his stepsister behaved like an educated girl from a good family is a problem that he did not want to address. It means taking care not to open your legs to anyone and that is something that no kid wants to do. Until he catches her showing her ass for social media and decides that her work deserves compensation. To give her away, of course. So he thinks it’s a good idea to put his cock in her mouth so she can give him a blowjob and she does it. Then he starts fucking her pussy and that way, after cumming, she knows he likes to take care of her.