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 girl looks a lot like my wife. My wife also loves to fuck in the doggy style position. So I had no trouble imagining myself in the boyfriend's shoes in the video. Beautiful sex in the video, sincere.