Before I say anything, I feel it is important to say that I have no training in this field.
Try to see that your niece and your cousin and potentially yourself get some therapy after what you discovered. This is damaging to each of you guys.
Pedophiles are great at manipulation. It may be good to help your niece to understand that she was being used for the perverts own benefit. If you choose to do this, I would suggest this being done in an environment with somebody trained with this type of trauma. So they will be able to understand the potential cues she has been giving.
An individual trained in these types of situations likely would be able to understand the types of manipulation that were being used used and help your niece understand.
I can tell you already have been, but continue to be there for your niece and your cousin.