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 my daughter has naughty conversations with her father, encouraging him in every possible way to fuck her, it is almost impossible to keep within the bounds of propriety. And she promises him pubes like her mother's. So when she took his dick in her mouth, he quickly relented. And soon poured his cum all over her sweet little bud. Cool theme.