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.
At first I thought that the grandfather would die at the end of it, but it turned out to be the opposite: he fucked the poor girl and poured a bucket of sperm in her pussy as well. Of course practically all the work the girl did by herself, but grandpa was also on top of it: at that age many of them can't get a hard-on at all. The girl sucks amazingly: swallows the whole cock without a problem, I would fuck her myself!
When the huge black phallus enters her pussy, the white chick feels euphoric about what is happening. Her opinion of herself rises to the sky. Getting a big negro to get on top of her is a reason to be proud in front of her girlfriends!