Sydney

I indicated to Kirsty that she should be quiet, which she did so, also stopping her oscillation. I reached over and turned on The Phone and started the second app (the one that made people do what they were told) before laying it back on the bedside table.

"Kirsty" I whispered "Tell whoever is at the door to come in"

"Come in" said Kirsty at the door which moments later opened to reveal Sydney, still in her bikini. The moment she did her eyes went wide "W-Wha-What-What!" she stammered

"Don't say anything Sydney" I said loud enough for her to hear, "just come in, close the door and sit down on that chair, facing us." I indicated the chair in front of Kirsty's desk and Sydney did as requested a look halfway shocked, half way resentful. I got the feeling she wished she was in her sisters place, which made sense given that I had made her fantasise about me earlier. I looked up at Kirsty and saw that she must have realised the same thing since she was looking smug.

I decided to teach Kirsty another lesson.

"Sydney" I said "Feel really aroused by the sight of your little sister having sex with me. Masturbate and imagine you are in her position, experiencing everything she is. Tell us when you are about to climax."

"Kirsty" I said "Keep riding my cock and feel even happier now that your sister is here to share this pleasure with you. Don't let yourself orgasm until Sydney says she is about to."

Kirsty resumed rocking back and forward on my cock, but now the enjoyment on her face was tinged with concentration as she tried to pace herself to make sure she didn't climax before Sydney did. Sydney, for her part, was watching intently with a look of sexual enjoyment on her face, her hands down her bikini pants rubbing her needy cunt.

This went on for several minutes and I made sure to play with all of Kirsty's erogenous zones for the benefit of both girls. During the entire time Kirsty gave little yelps with every thrust, and Sydney groaned and tried to keep her eyes open to watch us while obviously wanting to close them to better experience the feelings she was imagining her sister was enjoying.

Eventually I began to see a look of strain appearing on Kirsty's face as her climax became harder and harder to hold off. I was quite close to ordering her to release the pent up pressure inside her, without waiting for her sister, when Sydney groaned. "I'm going to cum! I'm going to cum!" and I watched Sydney's body shudder as felt Kirsty's shake and convulse in time with her pussy as both sisters experienced simultaneous orgasm. Although I could have held out longer I decided that joining their orgasms with my own was appropriate and shot my seed deep inside Kirsty.