I own my own house in London. A couple of years ago, my girlfriend moved in with me. We had an active and enjoyable sex life, though my sex drive is much higher than my girlfriend's, as she's usually only horny in the mornings. She sometimes said she'd be up for a threesome with another girl, or to watch me with another girl, but it never happened and we were fully monogamous.

Last year, my girlfriend's younger sister began studying at university in London, but is very shy, so struggled to make friends and got very lonely. My girlfriend asked if she could move in with us and stay in one of my spare rooms for a while, and I was happy to let her; she moved in in January this year. I was attracted to her, and we got on well, but I didn't initially do anything sexual as I didn't want to cheat on my girlfriend.

But a few months ago, my girlfriend told me that she knew I was attracted to her sister, and that her sister was attracted to me too. I didn't know what to say, but my girlfriend immediately said she wouldn't mind if I had sex with her sister too. I was surprised, but when my girlfriend confirmed she was serious, I said I'd like that. She spoke to her sister later that day, and confirmed back to me that she wanted sex with me too.

That evening I went to the sister's room, took her to the bed and started kissing her. She seemed nervous but excited. After making out for a while I slipped my hand down her tracksuit bottoms, and fingered her for a few minutes till she was moaning against my mouth. Then I took off her bottoms, lay her down and started eating her pussy. We were both very turned on, and she came quickly, then came again after I ate her for a few more minutes.

I couldn't wait any longer and I got my dick out , quickly put a condom on and slid inside her in missionary position. She moaned quietly as I fucked her and kissed her neck. I was so aroused that I came after only a couple of minutes, and I held her for a little while afterwards as we made out some more.

Since then, I've been regularly having sex with her as well as my girlfriend. I don't know how long the situation will last, but it's the hottest one I've been in