The bride's mother is not walked down the aisle during a typical Christian ceremony. The bride's mother is sat by the ushers that the bride has picked out before the ceremony begins.
The mother of the bride is sat last as a place of honor. If for some reason the bride's mother is not there, the bride may seat whoever has been the most like a mother to her in the place meant for her mother. This can include grandparents, stepparents, in-laws or even people outside of the traditional family structure. Some wedding ceremonies have the mother walking the daughter down the aisle instead of the father. This can be due to father's illness, death or personal reasons.