What should the aunt wear to a august wedding?

Depends on the time of day, place, and how formal. If it is an evening wedding and very formal then you want a dress that is formal and dressy. If it is an afternoon wedding and more casual you could wear a sundress. Your age also has something to do with the style of dress and you need to pick a color that looks good on you. Discuss the wedding with the bride or your sister/brother to find out the type of wedding it is and what they expect.