Persian culture customs

I am marrying into a Persian family. With the wedding weeks away, I was asked to allow the grooms cousin to walk down the aisle. She is a stranger to me and I have my bridal party set. What does the custom mean, that the oldest cousin must walk down the aisle? Does it have to do with her being unmarried? If she walks, does that mean she is a bridesmaid?
Thank you! I need the help.