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.
Considering the condition of the grandfather's device, I do not see anything surprising that the granddaughter let him clean her hole (I would even say, she begged to do it herself, apparently it itched a lot).