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.
Always said that a juicy body and a fun-loving woman can overshadow a young woman in terms of sex appeal! And what - quite a working body and knows what she wants from men. At the same time she knows how to satisfy any man's needs. In my opinion these women are perfect for sex!