Welcome to the QB
I hope this link to a guide using Pellon products is helpful. My personal preference is to use fusible vs sew-in interfacing but either should work just fine. I agree with Onebyone to use some type of featherweight so as not to add any/much weight to the quilt.
FYI - I use the often available coupons and buy at JoAnn Fabrics.
http://www.joann.com/on/demandware.s...acingGuide.pdf
Be sure to ask any questions you may have here as there's a wealth of wisdom eagerly shared by many and please post pictures of your progress! It's so nice that you're doing this for your sister-in-law.
Christine