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.
That is the kind of wife you should have - you can play cards with your friend in front of her and put her under him. And most importantly, she is not ashamed of it - hospitality is our strong point!