Escorting Of Grandparents

I need help! It is getting close to my daughter`s wedding--a couple of weeks. We are having a discussion that I need the proper etiquette answer to. My mother and father are divorced--the bride`s maternal grandparents. My mother is coming alone. She is remarried but separated. My father is also remarried and he and my step-mother will be attending. This is where the question comes in. I say that my mother should be escorted down the aisle and so should my step-mother. She isn`t so certain about this. I think that it shows respect even though my step-mother isn`t her blood relative, she is my father`s wife. How do you go about ushering the grandparents in? Does my mother come first, followed by my step-mother? And where does my father come in? She also has one grandparent living on her father`s side--his mother. What is the proper entrance for the grandparents? We need to know the proper way before messing up the wedding rehearsal. My future son-in-law works the church music for a lot of weddings. He went to a rehearsal the other night and said that their rehearsal wouldn`t be like that. No one knew where they were supposed to stand, play music, etc. I would like theirs to be better. Any help would be appreciated. Thanks.