I think you are making too big of a deal with this by over analyzing your daughter's teenage personality.
It is somewhat normal for children to be curious about what the other gender looks like. I remember the times we played "doctor" when I was a child. No one was tramatized and all of the girls seem to have grown into normal adults.
The flip side of this is; is your son telling the truth about how far he went? I find it odd that a 20yo talks to his mother, the mother of the girl also, about what he did to his sister sexualy when he was 10yo. How did this even come up?
You should file this under 'things children do' and forget it.