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.
What a good introduction to the girlfriend's parents. Although the stepmother is not her own mother. Still, she also decided to do her part in raising her stepson. The method she chose, it is true, is not the most popular - I have sex education. But I think it's a pretty brave decision. Considering that she is not his own mother, it cannot be considered incest; on the other hand, for this lady's husband, it cannot be called treason. Since it is his own son. Everybody wins!
Beautiful girl fucked.