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.
The son was peeping at his mother in the shower. She caught him and decided to give him a sex lesson. Cunny, blowjob, and fuck on the floor. She explained everything clearly.