Go to the menu, then settings on the Kindle, make sure it's deregistered, and register it on your
amazon account. (that's the quick short answer from hubby who is distracted and studying)
Once you have it registered to your account and turn the wireless connection on, the books from her account will come off of the Kindle. However, you can always go back into her account (if she gives you her account info)...Kindles can be registered and unregistered over and over. It's what my brother and his wife do with her parents' account.
Or you could just keep it registered on stepmom's account, but have stepmom put your CC as payment for the Kindle that is now yours...they've been changing and increasing the payment functionality for Kindles in this last year, and hubby (who is a tester for them) says it's possible, but again, he's distracted and studying so I'm not going to ask for the absolute detailed instructions right now.
But if you don't want to have your stepmom see what you're reading, or she doesn't want you to be in her account...just know that her books will fall off the device (but stay on her amazon account, since she bought them and owns them) once you put it on your account and turn the wireless on. (turning the wireless connection off when you're not actively downloading something keeps the battery life nice and long, FYI)