To me the definition of a style icon is one that has timeless style, and a desire to dress according to their particular taste, rather than the way society or trends state.
On that note there is no doubt that the Olsen twins are not style icons. They have influenced the way many people dress, from the bohemian layers they often are wearing (even in the summer!), to their constant dressing of fur and those round sunglasses! With their luxury fashion label 'The Row' it is evident that they have successfully made the transition from child stars to accomplished fashion designers, a road many budding designers fall on.
The reason that I personally love their style so much is that they are not trying to be anyone else. They never seem to follow the trends, they just go out and wear their own thing. In a few seasons time those things seem to be in trend anyways, whether or not under the Olsen's influence!
To get the "Olsen" look just think messy up do's, a lot of black, long flowing dresses, circle sunglasses, oversize bags and piles of scarves, cardigans and leather.