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.
When you have a young stepmother, and even with a butterfly on her leg, it's a matter of honor for the stepson to put a piston in her. Let the bitch know her place in the house.