Hi Celeste! :)
Your sister could always do what my mom used to do when she'd make us really fancy dresses out of fabrics such as chiffon where you could see through the fabric and she didn't want the raw edges to show. I think she called them "French seams" but don't quote me on that :wink:
Anyway, you simply make a narrow seam first by placing the wrong sides together, and then you re-do that same seam by making a slightly deeper seam with the right sides together. That hides all the raw edges on the inside. She could even go one step further and press that "seam" flat with a good hot iron, and then top stitch down the outside edge on both sides making it look and feel virtually seamless. Hope that made some kinda sense! :D
Try it out on some scrap fabric you have at home and you'll see what I mean.