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.
Well, he fucked his sister in the mouth, as it should be, and by the looks of it, it's not the first time.