I agree with FF. Do a 'tug test' on one of the blocks. Hold a block by its sides (not on the bias) and give a firm tug. If the fabric shreds or tears, you would need to stabilize them. I would use a light weight or feather weight iron on interfacing. If they are OK, then I would just sew them up.
You can set them on point, as you have them displayed with or without sashing (I would get a repro 30's for the sashing). You could also alternate them with plain blocks, either muslin or a print. If you alternate them, you would probably have enough for 2 throws. One for you and one for your sister.
One more alternative is to see if you can get some fabric that matches the background. Cut 1.5 to 2" strips and sew them on all 4 sides of the blocks, then square them up. This gives you more area to work with, and you can get the butterflies in the same place on every block.
One final idea on the crinkly/ crispy. there may be paper in back of the applique. Some people used to use paper for needle turn. you could check this out with a straight pin.
Last edited by PaperPrincess; 02-06-2016 at 09:19 AM.