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.
That's how the mother took advantage of the situation. She put her ass on the line for her daughter. Now they'll mistakenly think her daughter's a whore. And even her classmates in the bathroom will pull it on her.