So sorry for your loss. You are taking on a very intense project, and will be so happy with the results when you are done. I would suggest making a lap sized or a wall hanging size so it can be displayed rather than used. Then you will be able to make several if you want for her children as a keepsake. Loosing a sister is such a great sorrow. Take your time and I'm sure whatever you decide to do will be beautiful.
Sue