Negroes, mulattoes like to spank white girls in all holes. The fact that this brunette is also a charming girl - gives a glamour to what is going on. And he's just using her as a device to jerk off his dick and lick his balls. He uses it and then he flushes it. And she's satisfied.
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.
It's a definite experience for the couple and an opportunity to diversify their sex life and try something new. But I was more confused by the lack of people behind the showcase, no one is interested in this from them?