It appears to be much less full than the Fairy Tale, which is designed to be very full. The skirt is still basically a rectangle, but just not as wide a rectangle as the pattern has. I’d recommend that you add about 12″-14″ to your sister’s hip measurement and use that total measurement as the width of the rectangle. In other words, if her hip is 36″, make the full rectangle 48″-50″, or 24″-25″ for the front rectangle and 24″-25″ for the back rectangle (plus seam allowances).
If anyone else is looking for that same silhouette, you could certainly achieve it with the Lisette Continental Dress (http://oliverands.com/lisette-patterns/OLV-U02209.html). The skirt is already not very full but could be made still less full. And if you do a dart manipulation (http://www.creativebug.com/workshops/dart-manipulation), you could move them into the side seam the way the dress in the link does.
I hope that helps!