My only attendant will be my sister. Since she's the only one, I told her to pick a dress that she thinks is pretty and feels comfortable in, and the only stipulation is that it has be some shade of aqua/teal/turquoise. Where should she shop? We're having a formal wedding, so I'd like her gown to be formal as we'll. She can't do strapless and would even prefer sleeves. Suggestions?