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 do not know about the indefatigable brother, I think he got tired) Sisters certainly are all on the positive. The way they got caught by their mother and the brother hid, it was well thought out. But then when they went on and the mother, or whoever she is I don't know, was sitting next to them, I didn't understand why they did that. It was nice to watch, especially the sisters, the brother was kind of passive in the clip, he was almost never even shown.