I think you will be happier with the sewing room on the main floor, I know I would. The bathroom, kitchen, and laundry area would be closer. I know how hard it is to go up and down stairs and it only gets harder as you age. I wouldn't even want my bedroom on the upper level, and as Mariposa said, stairs aren't a problem for your DGD. Also, you will get more done if you can pop in whenever you feel the urge to sew.
Sue