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.
Great student, passed the exam with flying colors. Taking advantage of her gorgeous forms, she seduced the instructor, and he could not resist. First gave him a beautiful blowjob, then saddled him.