My sister and I are close, as I've said. We both know the other has had sex and has jokingly said something to the other before (we were out and her t-shirt fell down as she bent over revealing a massive hickey - some silly innuendo and we moved on; I fell pregnant the month I moved in to a new house - some silly joke about christening christening house etc). However we absolutely do not discuss sex, preferences or our vaginas in that much detail with each other.
It's just wrong.