We were staying at my then girlfriends (now wife) sister's flat in London. Both my girlfriend and her sister decided that they would go out shopping - I decided to stay in the flat. After about an half an hour I became a little bored and decided to have a look around the flat. Naturally I ended up in the bedroom. I went through a couple of drawers and found her underwear drawer and her bras. This started to get me a little turned on. She is a big girl up top 34DD with nipples that are hard to hide. So I decided to have a quick wank. I got a couple of bras out and put them on the bed and started to stroke. However the now sister-in-law had forgotten some blouse that she wanted to return and returned to the flat - I did not hear her and she was at the bedroom door watching me! I had my back to the door, the first I knew of her being there was when she came and sat on the bed! I started to mumble something about being sorry..but she started to undo her blouse and before i knew it her bra was off. She then said "Well if you are going to wank you should have some better material, how about these." I dont know why but I carried on staring at her tits until I came in my hand. She then got dressed and as she left the only other thing she said was "Feel better now?" Neither of us mentioned it again or have done anything like it since. That was 10 years ago and I still have a crafty wank thinking about it now.