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.
This brunette is very bright and liberated, and it's a pleasure to have sex with these kinds of girls. The fact that she has developed her anus like that and then moves it on her own at a brisk pace is rare at all. If she moans sweetly, it means that she gets a high from it, and not just does the job.