Have your cousin place a "dummy box" on her porch with a note in it, stating please bring back a quilt made for a cancer patient. However, I would place a camera trained on the porch with the dummy box. We did this when our daughter's car was keyed. We caught the "trash" the nex time he tried to damage the car. These days you have to be your own police. I would also place a theft complaint with the Postmaster on both ends of your mailing experience. So sorry for your pain.
D in TX