A little humor in porn is only a plus.
This guy who is tied up, he appears in many videos, I think, as this simpleton who gets cheated on by his girlfriend. Just look at his face, it simultaneously expresses frustration, helplessness and cowardice. I wouldn't be surprised if, after the lover left and the girl untied him, she only had to say a few sweet-faced phrases for this cuckold to forgive her.
To tell you the truth, considering the age of the brother and sister, it is quite normal that the brother became aroused by the sight of the naked girl in front of him. Maybe what happened next is not part of the plans of normality, but tell me honestly, would you resist such a dark-haired beauty? That's what I mean.