iTunes help switching songs from one user to another
Go to Home > Music and copy the "iTunes" folder to an external drive.

If you don't have an external drive big enough, you can move it to Macintosh HD > Users > (Your User Name) > Public > Dropbox. (You can copy it directly to your music file from another user, as those folders are only editable when you are in your account).

Login to your user, and move the "iTunes" folder from wherever it is to Home > Music. Open iTunes: your sister's library should be loaded.

PSS: I by no means am advocating the copy of copyrighted content with this post. I imply your music is legally downloaded, and you are moving the library with the intent of organizing it and not duplicating it in any way. Don't do illegal stuff!