As far as I have ever been aware, it has always been a rule of etiquette for a female guest to NEVER wear white when attending a wedding in the United States. It has always been my understanding that the bride should be the only female dressed in white.
And still our answer, forever and always, is no. No, you cannot wear white to a wedding. Period.
The only wedding that I have ever witnessed outside of the United States was the televised wedding of Prince William and Kate Middleton in England. Kate Middleton's sister wore a white-(ish) colored bridesmaid dress. I don't recall any breaches in the rules of etiquette being discussed when this happened.
Would a female wedding guest dressed in white only be breaking a rule of etiquette in the United States?