What should a 77 year old grandmother wear to granddaughter's 5 p.m wedding?

My granddaughter is getting married October 1, 2011 at 5 p.m outside a hotel by the river in Sacramento CA Bridesmaid are wearing burgundy, mother of the bride is wearing a rose petal colored long dress. I would like a dress with a jacket, duster jacket or shawl. I am 77 yrs old, 5 feet tall, 120 pounds. Short brown hair. I am a Autumn person.