With the book open, press the menu key. Select "go to" and a text box will open giving the choices of "cover", "beginning", "location" and "table of contents". Choose the one you like.
The software wasn't designed for multiple people sharing the same Kindle. It was designed for one person who might want to sync with other Kindles on their account or Kindle apps. Your wife would be able to read her Kindle to a certain point, then pick up where she left off on her smart phone (or whatever) via the Kindle app.