In fact, it's a proven fact. No one would refuse such a training session on boxing, see how she was furiously sucking his big dick, and she seems to be enjoying it as well. In general I think that such a fuck will be a norm for them now, because they are unlikely to stop at the received emotions, they will want more and more, and there more and more, we just have to watch.
If I found out my sister was a slut, I would have pulled her in the asshole, too. For these guys, getting their asses handed to them is commonplace. Although for the look, they break for a few minutes, but when a couple of drops of lube appear - they quickly sit down anally on the phallus. So this little sister is nice to feel like a courtesan, to show off all her skills in front of her brother, to let him cum in her ass. Every bitch dreams of being the best at it.