A father should always know what his daughter is doing. Even in the bathroom. For educational purposes, of course. The main thing is that she does nothing wrong. So he went in to check. The fact that she was masturbating was so pleasant and exciting that he decided to introduce her to even more enjoyable games. Well, what loving father would refuse to let his adult daughter suck his cock? And developing her anal pleasure - just part of the duty of a parent! )
And why the girlfriend was so turned on - he was her half-brother, not her own. And who's gonna train his sister, a stranger's uncle? No, only your own should be trusted with her pussy.