I would thank her for asking you to make the quilt and let her know you were flattered that she asked you. Then be firm in saying, I will have to decline your kind request, because I am not able to make All my cousins a quilt and would not want to hurt anyone's feelings. I wish there were more than one me.
Then say with a smile....I would love to help you learn to make a simple quilt. If she lives close, she can come to your house. If she is far away, she can take a class in her town or use a tutorial online. If she has zero interest in trying, refer her to someone that makes quilts for a living...etsy or ?
Good luck!