I think discussing your suspicions with a therapist is a very good idea. Also, maintaining a constant and open dialogue with your niece is also very important.
Mark, she's actually been through extensive therapy and was released about 6 months ago as healthy. That's part of what makes this so tricky because as we all know those that have been molested are easy prey and since her the therapy sessions are over no more reporting or observing by a therapist is necessary. However, I think it may be a good idea for me to speak to a therapist about my suspicions. I just joined this site, saw similar questions and decided to post mine her as well. Thanks for your feedback.
It was her Father's father who molested her, her mother is my sister
If your niece was sexually abused, she should be receiving mental health counseling. Kids who were or are being sexually abused typically have boundary issues to no fault of their own. That is why therapy is so important. Regarding your concerns of suspicious behavior, this issue should be discussed with the therapist.
Was the grandfather who molested her, your husband's father? Do you know if your husband was ever molested? I'd be more worried and suspicious if so.