You found your little sister asleep, this is your chance to touch her where you always wanted to, but if she wakes up, you're dead, or maybe not =)
You can undress her and move her legs to different positions to do this - use the "hand" tool. You can use all three tools at the same time if you put them in automatic mode, but this can quickly Wake up your sister, so don't forget to raise her sleep level.
If you do everything right and give her maximum pleasure, you will be rewarded.