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 are you gonna do, son, with a mother like that? She's sucked more than that! She said: Come on! - then unzip and stick it in. When a bitch has a fire between her legs, she don't care if you grow a pussy or not. She'll grow it as she goes along.