An IMDb contributor likely submitted her solo acting credit as an Actor in error. If you have an IMDb user account, you can submit an update to correct the error. If not, then another contributor or an IMDb representative can submit a correction.
The link below is from a previous GetSatisfaction discussion on how to correct the error.
I went ahead and submitted a gender for your niece. Give it 24-48 hours and it should be accepted. This generally happens when someone is credited as crew member before any acting credits and it defaults to male and then the acting credits are added and end up under "actor" instead of "actress".
