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 naive girl cannot haggle yet, but she already has the makings of a businesswoman. The proposed amount of 500 euros will help her get a taste for the sweet life and get to know different dicks better. The Ukrainian girl is very appetizing and determined. Once the opportunity to fuck near the car has come up, then you can't pass up the chance. Her wet pussy will only thank her owner.
And why the muscular guy didn't want to take a walk in the rain, but sent his muscular hottie there, that's not very gentlemanly.