Do you have any of the muslin left that is unwashed?
If yes, you could measure a piece of it "before" - soak it in hot water and let it dry on a towel or hang it if you don't want to put it in a dryer - iron it - and then measure it "after."
You could do the same thing with cold water - just for comparison purposes.
If you do, let us know the results.
I have had "pre-shrunk" muslin shrink a LOT!!!!!
Why in the world would your cousin NOT want you to wash the fabrics before cutting them?
I wash EVERYTHING dunkable before cutting it now.
I had a polyester double knit shrink about four inches in length - and it was only about a yard long to begin with. It was yellow - which has nothing to do with anything - but I remember it well, because I was very surprised that it shrank that much.
I measured hundreds of pieces before and after washing them to check for shrinkage. Some (a very few) did not change size, some shrank maybe an inch or two in length or width, some shrank over two inches in width and/or length per yard.
I am not able to tell - just by looking - how a fabric will behave once it is submerged in water and dried again.