It's not even quite clear whether the brother helped his sister, pleasured her, or taught her. Either way, she should be grateful to him after a fuck like that. It's funny to see how he uses both his phallus and her toys to develop his sister's anal hole. It's bright and exciting, and you can see the details that are so valuable in a process like this. I wonder what he will turn her on with next time.
How do you explain to a blonde that a coworker has a wife at home and is a good family man? She just doesn't get it! It is easier to cum in her mouth than to explain it. So his conscience can be calm - he did not cheat, he just bought the girl a cocktail. And he put it in her pussy just to make conversation - fuck me, das ist fantastisch. Let the blonde think it was her charm that worked and make her feel happy.
If a chick offers to tie you up, her main desire is to fuck your mouth with her pussy. She's only interested in your dick because she's not interested. And even if you cum, she won't suck it. So the blonde just humiliates the guy by squirting in his mouth.