I have been involved in making charity quilts where the guild members were to each supply a 12 1/2" block. There were about 100 very experienced quilters who made blocks that (unfinished) varied from 11 3/4 to 13 1/2. I put sashing around the blocks and made some very lovely quilts.
Since what you are doing is going to be a cherished keepsake, accept that things are off and do what you can to make them special. The quilt will be lovely.