雲雨 (yun2yu4: clouds and rain) are a part of Daoist sexual manuals, and many of the classical idioms that Chinese speakers use for sex come from those texts.
激凸 (ji1tu1: the excited protrusion) refers to nipples and hard-ons that can be seen through the clothing, so "pitched tent," for instance. I assume that it works for "camel toe," as well, but I've never seen it.
But if you really want to get into that, you can find Chao's book on it almost everywhere: https://rs353p6.rapidshare.com/files/34 ... School.rar
Even 小弟弟 and 小妹妹 don't seem all that vulgar to me, but it's hard to make me flinch in the Internet 2.0 era.