My mom says it's only right, but is it?

My mother and father are divorced and have both remarried. I am ordering wedding corsages for my mom, grandmothers, and the groom's mom; my question is, should I also order a corsage for my stepfather's mom even though our relationship is not one of grandmother/granddaughter? My mom says it's only right, but is it? I don't feel she deserves the recognition of a grandmother to me since I don't consider her one.