Let’s assume she is telling the truth.

If this happened between ages 9-12, I’d be looking at the family as a whole to determine how this happened at such a young age and for so long with out any adult knowing. There wasn’t the internet where information about such topics is freely avalible. Someone planted the seeds and some one else failed to educate the kids about personal space while also somehow making the girl not want to tell her parents “hey my brother invented a wired new game!”.

I’ll also acknowledge that the girl may have been “consenting” to secret play time with out really knowing what was going on. At the time all was fine, but retrospectively all was clearly not fine.

Ultimately it’s a he said, she said fight and we will never know the truth.