As an usher you should actually be in a tux. You are a member of the wedding party, and if they are in tuxes you should be too. If they are not in tuxes the people paying for the wedding (usually the bride's family) will/should indicate what attire is appropriate for the wedding party.

On an aside - not indicting just curious - how is it that she's never seen her favorite uncle in person - the one who wears a kilt - kilted? Again just curious.