I am a male and at the time I was 15. My only sister was 17. She had her friend over that is very very good looking. So it was getting late my sister and her friend where down stairs so I went to go see what was up. I walked by my sister and gave her a little tickle. She said hey and grabed me and threw me on the Lap of her good looking friend. So I was sitting with my legs around hers facing towards her. She said "I'll hold him down, go get something to tickle him bad". My sister ran off and I was left there. She smiled her beatiful smile and started quickely tickiling my armpits. I don't know what happened I was laughing and I was fine but then... The tickiling was arousing me and the position I was on her with her short shorts. I had sweats on and all of a sudden they started to tighten. I had a massive boner in her lap. She looked down and her mouth went wide open and she smiled at the same time. I broke free of her grip and lost balance and fell off her and landed on my back. She just eyed my boner and said "oh my..." and put her hand over her mouth. I got up and ran up stairs with my erection.

Now when ever she sees me sometimes she just smirks, but other times she stinks her finger down her pants and straight out so it looks like she has a boner. I do my best to ignor.