I hope you will be able to finish this by hand quilting. I would stay with the 1/4 in from both seam lines as she was doing. I wouldn't worry about trying to match her stitch length exactly. all beginner hand quilters do get better as they go along, you may find by the time you are finished you have mastered very tiny stitches. You will also have in the quilt lable that your grandmother started the quilting and it was finished by you, so that will explain the difference in the stitching.
Good luck to you, I am sure you mother is going to love you very much for doing this.