All logical: he takes off the brunette, she takes off his pants, and they are each engaged in their usual business. The girl, by the way, will go far in the modeling business, where the ability to completely swallow a big cock, while smiling at it, is highly valued. Give a deep blowjob with a friendly face, this is memorable and the next time you choose a model, she will automatically be at the top of the list!
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.
I'm screaming at you, of course.