You can easily import both your Bookshelf and your sister's into the new account. You download your Bookshelf to an Excel spreadsheet. This is explained here in Help. Your sister does the same. Then copy and paste the list of ISBN numbers in the spreadsheet to the new membership, using Import Books, accessed under the My Bookshelf tab. If you and your sister own the same books those will only import once.
You won't be able to do the import for magazines since they do not have a unique identifying number, but magazines are easy to add in one go, by entering the date range of the magazines you own, explained here in Help.