I am sorry to hear about the passing from your mother in law. My symphaties for you and the family. What a beautiful age she lived to be, 93. You got a big job ahead of you but I know this will be very special to both you and the recipients.
I make quilts out of baby clothes and use light weight interfacing on the back of the T-shirt and sweat shirt material. I would suggest also considering breaking up fabric types like sweat shirts in one quilt, a jean quilt and a T-shirt quilt. Definitely consider not using batting with the jean and sweat shirt quilts and a light weight with the T-shirt quilt. These quilts tend to get heavy. I stiple them all over to help keep everything in place. What is hard too is ironing on the interfacing when you have those plastic decals and scences on the sweat shirt/T-shirt. They tend to melt!
Here is one of the baby quilts I made. The second picture from the top.
http://www.quiltingboard.com/t-52466-1.htm
Since these are clothes from an adult it should be easier to cut larger squares. I would suggest adding sashing to reduce the extreme bulk created sewing thick fabric from clothing onto thick fabric.