Considering how much they'd been drinking, I'm not surprised they had the idea to have a threesome. Especially since the mom is such a badass. Kissing your daughter in front of her boyfriend meant offering yourself up as a pussy for copulation. And the guy took advantage of that offer by banging both of them. He even shared his sperm with his mother when he came between his girlfriend's legs. Damn, that's fair!
The sister is really uninhibited. Judging by the reaction of his brother and his unwillingness to break away from the game, his sister is already bored with him to say the least.