The sisters are those pussies, which you try to satisfy, see how he fucked them, and they do not give a damn, go around smiling. I think that all was filmed very cool, it is clear that the picture worked hard, and the main character perfectly grilled these young chicks, who apparently have not had sex for a long time, as they give him a good hand, the cock came to their liking, moaning like wild.
I love this pose too.