I agree with cjsews. I had a bunch of blocks that finished at 8 inches and 9 inches. I added borders to bring them all out to the same size--12 inches finished in this case. I then added sashing between the blocks. If anyone ever noticed the difference in the sizes of the original blocks, they never said anything. And that quilt took a first place ribbon at a couple of fairs!!
They are lovely blocks and you are a lovely person to help out your mother in law.