EBay has a very pretty necklace- "LBB Boutique Princess CINDERELLA Crystal CHARM Necklace" with a buy it now price of about $27.50 including shipping. You could give this to her as a token gift, and then choose something from their registry for both of them together. There's also a guest book with a Cinderella Carriage frame on the front, if their using a Cinderella theme. One more idea that may be a nice compromise between Purely Cinderella, but not part of the registry either- Get a Cinderella themed frame, and but cash in it. I frame cash gifts ALL the time. And most newly weds could really use the extra money.
Also, if they aren't already been living on there own, here's a GREAT house warming party idea. Send a Recipe card out with the invitations. Have the guests bring their favorite recipe written on the card, something they'd forgot they'd need when they got their first place (like toilet paper, a plunger, cooking spray...) and either a board game (if your sis and her fiancé are into them) or a framed picture of each guests’ family. You get SOOOOO many house wares at the bridal shower and for the wedding, but every woman needs a collection of taste test recipes, and the forgotten basics, and all homes feel warmer (hence House-Warming) with game to play and pictures to see.
Hope this helps! –Janell-