Tradition plays a big role in the wedding procession. If the mother-of-the-bride is not part of the bridal procession, she is usually the last person seated before the ceremony begins. In Christian wedding ceremonies, she sits on the left side in the front row. In Jewish ceremonies, it's tradition for the bride's mother and father to process up the aisle with their daughter and remain under the huppah through the wedding ceremony.
However, that's just tradition, and today's families don't necessarily always follow tradition. Even if you follow the specific rules your faith dictates for the actual ceremony, you can always bend the rules a bit for the processional. For example, if your ceremony is Christian, you could still ask your mother to join your father in walking you down the aisle.