Because he is his cousin, he only lets himself be fucked in the ass
She may be trying not to fall into the sin of fornication, or she may simply want to avoid getting pregnant. Whatever the case, it doesn’t matter because in any case, your solution is ingenious and also innovative. Or maybe it’s old, but it still doesn’t matter. What matters, more than what he does, is seeing how he does it. Because this girl, being her cousin, only lets herself be fucked in the ass. And after giving him a cock suck and making him stand firm, she herself incites him to give her an anal fuck. And that’s something this kid can’t reject like no one would in his place.