I am having awesome secret s*x with my girlfriend’s sister. We know it’s wrong but just can’t help ourselves. I invited her back to my place for a drink after we had been at a mutual friend’s party.

My girlfriend is a nurse and was on a night shift that evening. I am 27 and she is 25. We have been together for six years and most of that time it’s been great but lately we haven’t been getting on and have been arguing quite a bit.

We are not physical in any way at the moment either. I think some of it is down to my girlfriend working shifts — we can never plan anything together. I have always got on well with her sister. She is 23, sexy and really good fun.

She agreed to come back to my flat that night and on the way, as we were talking and laughing I realised that we had a real connection. We couldn’t stop flirting all the way there.

It felt really good and I jokingly said that it felt like our first date. When we got back home we ended up finishing a bottle of wine. I got up to make some coffee to try to sober up, but first I reached over to give her a kiss. She kissed me back and one thing led to another.

We ended up having s*x. It was out of this world but I felt so guilty afterwards, especially when I thought of my girlfriend working so hard at the hospital.

Her sister and I have been in contact constantly since that night and have had s*x a number of times. She says she has feelings for me and I’m the same. I am falling for her big-time.

The only problem is the connection between my girlfriend and her sister. How do I tell my girlfriend that I no longer want to be with her? What should I do?