This is a true story that happened to me a couple of years ago, it could have been a dream but wasn't!
Let me give you some background first. I was 28, my girlfriend Sarah was 24, she was around 5'1, long blond hair, 34b very firm breasts and was stunning, a real catch as they say. She was one of 3 gorgeous sisters, one of which, Rebecca, is also part of this story. Rebecca was 19, also blond, great tight body with 36b/c tits. She was a virgin, all the family knew this and she was proud to be, on one night out I remember the girls talking about "big knickers" Rebecca said she purposely wore big ones so she wouldn't get carried away, in fact at that time she had only ever had one boyfriend. Although obviously not twins the girls looked very much alike.
I had been seeing Sarah for about 18months, we were living together and although she didn't know it (at that time) we were about to be engaged. During the 18 months we had been together I had got to know Sarah's sisters pretty well and I always had a soft spot for her youngest Rebecca, and certainly when I first started seeing Sarah I thought that Rebecca was jealous, when we were at her house she would often come downstairs in just her towel after just having a bath and sit next to me, although nothing happened and I never saw anything I could tell, or maybe I hoped there was something to it.
Anyway, one weekend Sarah and myself were visiting her family, we were spending Friday night in and we would be going out on the Saturday. On the Friday we really just caught up with her family watching TV etc until around 10pm when I said I wanted to go to bed, so Sarah and I went up stairs to the spare room leaving Sarah's mum and dad and Rebecca down stairs. As I lay on the bed (fully clothed) Sarah went to bath room. A few minutes later the door opened and I felt what I thought was Sarah climb on & straddle me, however as I moved my hands up from her waist to her breasts I could tell something was different..... it wasn't Sarah it was Rebecca dressed in Sarah's top to confuse me, as I looked over Rebecca's shoulder I saw Sarah peering in through the door laughing her head off.