Evening summer attire, basically just means evening attire, but bare shoulders, open-toed shoes and men without jackets are expected/accepted. Think of what you would wear to a cocktail party, then remove the outer layer. Dress shoes are still the right footware choice, so make sure your clothes are dressy enough to go with shiny shoes :)
Hubby and boys: White dress shirt, tie, dress pants, dress shoes (the boys might be able to get away with khaki's, but they will be under dressed... hubby should definitely wear real slacks).
You: Cocktail dress (floral or bright print would be pretty), bare legs, pumps or dress sandals. A more structured, non casual sundress could also do the trick.
HTH
T.