How can I change the iTunes login for my iPad?
This is easily done form the iPad itself, you can just enter the iTunes or AppStore app on the iPad, scroll to the bottom of the frontpage, and press the button labeled "Apple ID:[your email here]". A box will pop up, where you can choose to log out of the currently logged in Apple ID. You will then be able to press the same button again, and then log in with your wife's ID.

Good luck :)