Resetting Kindle Fire HDX?

My aunt recently gave me her Fire HDX, but I’ve only recently gotten around to using.

Come to find out, it has parental controls on it (she put them on for her kids). After some googling, I found out that I can do a factory reset to get that sorted out…

Only to run into another problem- the device is registered to her amazon account, which she no longer uses (she uses hubby’s) and has forgotten the password…

The password I need to reset the device to factory settings- I’ve already tried to get her to recover the account, but she has given up on it.

Any advice how I can reset this thing?