If it really is your phone, you can take it in to your provider's store and they can unlock it for you. They'll verify your ID (so take it with you) and make sure the phone really belongs to you before they do anything, however.
If she completely took over your phone, they may want to wipe the OS and start over, so you may lose your info that was on there if you don't have a back up.
Or, you can always ask your aunt for the PW, since you got your phone back.