How Many?
I have no idea just how many shirts it would take to make a quilt. it depends a lot on the size and style of the shirt and the quality of the fabric. if the shirts are worn and thin in places it may be much harder to find usable fabric in them. Also the difference between and long and short sleeves makes a big difference. If she has a lot of good usable shirts you may be able to get a large quilt made but with a lesser number you may have material for only a smaller throw. There are also pillows which are made out of shirts which some people find very attractive. When my husband passed away i used some of his shirts to make memory bears for each of my children. Each bear took one long sleeved shirt to make. My children were very pleased to receive the bears and recently I made another bear for a family that had lost their father. I purchased my pattern directly from the Simplicity company and it works very well. Have your sister in law show you the shirts that she wishes you to use. You need to make sure that the material in the shirts is suitable to use in a quilt. I would not attempt to consent to making any particular quilt until I i knew just what materials I had to work with. Explore for ideas but in the meantime be sure to actually see the shirts so that you know what you will be working with.