Well, half-brothers and half-sisters are not related at all, so it can't be considered something bad or immoral. It's no surprise that an adult guy and girl, with no regular sexual partners and almost daily being around each other, suddenly felt attracted on a sexual level to each other. Considering that the girl liked it (the guy then no question), I think they will continue to practice this kind of thing from time to time.
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.
The pretty brunette always had something to pay, and the trainer took advantage of that. And he did the right thing. Sex prolongs life.