If I found out my sister was a slut, I would have pulled her in the asshole, too. For these guys, getting their asses handed to them is commonplace. Although for the look, they break for a few minutes, but when a couple of drops of lube appear - they quickly sit down anally on the phallus. So this little sister is nice to feel like a courtesan, to show off all her skills in front of her brother, to let him cum in her ass. Every bitch dreams of being the best at it.
And why would the grandfather resist? When else would he have such an opportunity. Besides, it's always easier with a temperamental woman - she gets herself turned on and therefore comes much faster. You would have tried to bring to orgasm phlegmatic fat girl with a huge hollow. That's the kind of woman you should run away from. I've been in the middle of one of those, you can't wish it on the enemy.