Not exactly but sort of. You can change the account that it's registered to at any time. It won't automatically delete the books that are already on there. For example, she can deregister her Kindle from her account, then register it to your account (via the Kindle settings or via the Manage Your Kindle page for your account) and access the books you have archived on your account, then register back to her account. It will screw up the syncing, though, so that's something to consider. You also won't be able to share books purchased from other sites this way, if that's an issue, but you can't do that with a shared account either.
It's kind of a pain if you're going to do it often, but as far as I know your only other options would be a shared account, or stripping the DRM and sharing them all from a folder on your computer.