My sister hadn't understood that teasing a relative was one thing, teasing a mere acquaintance was another. That morning, while I was in the garden playing with the cat, I saw her fooling around with the elderly neighbor, who, after half an hour of rubbing and double entendres, invited her to follow him into the house. She followed him fearlessly and defiantly, probably underestimating the virility of a man her age. After a few minutes, the voyeur in me urged me to approach the neighbor's house, and following the voices, I approached a window and saw them. Both were naked from the waist down, and while he was kissing her, he was rubbing his big, veiny cock between her thighs. At first, I watched excitedly, but then, seeing my sister trying to escape the man, I decided to intervene. I had no problem getting into the house since the front door had been left open, but when I reached the living room door, I realized my sister was no longer shy about the attentions of that man, who was on his knees with his head between her legs, licking her while playing with her ass with his hands. Then it was my sister's turn to lick and suck his cock, and seeing how much effort he put into it, she happily did so. With such treatment, I would have come twice already, unlike him, who seemed to be only just starting out. When he leaned my sister on the arm of the couch, I thought he wanted to fuck her, but her reaction when he placed his cock on hers told me he had other intentions.