If either of them ever starts comparing their time with the baby, don't argue with them, just ignore their complaints, if any such ever arise. Your own mother is special, of course. Her connection with you and with your baby is obvious. The connection between the other grandmother is, apparently, tenuous. See what you can do to make her feel comfortable with you and your baby.
Life is long, indeed forever, as far as the living are concerned.
Try to let this relationship be smooth and without conflict. As the mother of that baby, you have the responsibility of making her life comfortable. Carrying on a battle with an in law, be she step or not, will create a very uncomfortable environment for your child. I know you want better than that. Nothing is easy, but accepting this woman's relationship with your daughter will not be your most difficult challenge.