I was staying at my ex girlfriend's house and we were both still at sixth form at this point. She had a younger brother that went to the same school so he'd walk with us. Now her brother was already renown for being a oblivious on many accounts but this must of been the cherry. GF is on top and we're under the covers, her brother gives a courteous knock before barging in regardless. Gf panics and drops so he comes into see us sandwiched together. Any normal person would of known what's up. Now he's just standing there and we're having a normal conversation with him, all while my gf thinks it's funny to keep going just a little but not so much that he could see (very thick covers). He leaves and we continue the hanky panky. (Sorry) At this point I'm pretty close and I'm ready to start the day off with a blast...(sorry again). But, alas not if little bro can help it. He barges in for the second volume of stupid questions. This time we were not prepared. Caught in the moment my gf jumps off me this time, grabbing the covers and pulling them up with her to cover herself. They come up so far that I'm exposed completely. I promptly press myself against her to hide as much as possible, of course, it's hardly working. This time he still stands there as if nothing is amiss. We causally drop in that we were getting changed and that he needs to knock. He wonders out and makes a teenage grunt. At this point we're already late and I am blue balled for the rest of the day. God knows the amount of subtle "jokes" I made about having sex in the disablad toilet in school that day.

(We were 17 - 18 at the time, little bro was 13)