H.H.
If she can write, see if she will write it down or draw a picture of what is happening.
Rather than asking her when she does something and snapping "where did you learn that" try talking about it when she is not doing something inappropriate. Like if you are playing a game or something just casually bring it up, " remember when you grabbed my boob in the bathtub, why did you say that?" Just try different ways of bring it up and be very open about the conversation. Like "who has small boobs?" or "have you ever opened someone else's zipper?" It is best for her to not be ashamed about the things she is doing so she will tell you if something more is going on.