I don't know if you'll enjoy reading a NY Times profile of the Olsen Twins, but I was oddly fascinated.
Mary-Kate's contribution to the enterprise is a collector's knowledge. She has been buying vintage Lanvin and Givenchy, among other classic labels of the mid-20th century, for a number of years. (Unlike Ashley, Mary-Kate continues to act, having played, with a perfect semblance of haze and obfuscation, a born-again Christian drug dealer on the third season of "Weeds." This year she appeared opposite Ben Kingsley in the film "The Wackness.") Ashley is the more entrepreneurial, the one who will tell you how much she admires Steve Jobs and Bill Gates.