Changing iCloud accounts in order to sync Photostream pictures
Go to Settings.app > iCloud and Tap "Delete Account". When you do this it will give the option to keep iCloud contacts and calendars on your device. You will probably want to keep them on the iPhone. After deleting the account (this really only signs out of that iCloud account) sign back into your iCloud account.

From: this thread.

Keep in mind that the backups from her phone (if enabled) will now impact your storage amount.