The girls were looking for fun, riding in the car. Sometimes they got themselves excited. Apparently they wanted a new sensation, so they offered a threesome to a strange young, handsome guy. After some persuasion and conversation, he agreed and went straight to work. The girls hooked up with him, gave him blowjobs, rolling on top, while two were fucking, the third fondled the couple.
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's her name?