When we were young me and a friend tagteamed his sister repeatedly in their treehouse. We were young enough that we were trying to have a "boys only club", but she was a year or two younger and kept wanting to join in. So my friend agreed to let her in, but she had to show us her boobies. I thought he was joking, but she lifted up her shirt for us and I got a (mostly flat) eye-full. After that, in order to play in the treehouse, she'd have to give us the special "password" of showing her boobies or privates. But quickly we started asking for new and different things, like her completely naked, letting us squeeze her boobs or touching between her legs. It just escalated from there until treehouse time was exclusively to explore her sex parts and pump our wieners in her mouth and pussy. We were young enough we could keep playing all afternoon and sometimes fingered her pussy hole for hours. After summer vacation ended though we didn't have the time any more, and we started getting too old for treehouses.