I think you should tell her the truth. You have a lot of cousins and would not want them to feel bad because you made one for her.
You could also offer to make one for her and charge an amount that you would charge a stranger or else a' family' price. You could offer to teach her how to make a quilt. Of course, if you give a 'stranger' price or a family discount price or offer to teach her, other cousins may come looking for the same.