I'm a Sharapova fan, but she certainly isn't a worldwide superstar that some of her fans claim her to be. Well, not in England anyway.
You can certainly argue when the William's sisters first came on the scene, they had far more impact, notoriety and fame, than Sharapova's impact, even after winning that oft talked about Wimbledon vs Serena. They're not even comparable.
The Williams sisters were phenomenon's, along with their rivalry with Martina Hingis. It was great for tennis (interest, ratings, buzz, attendance), and sports on a whole. Those were the days man.
Even now, when the Williams sisters show up an event (more so Serena than Venus), they suck up all the life, buzz, predictions and coverage. Think about it. It's been this way despite the fact that they're basically part time players with other interest.
I'm sure if you were to ask the average sports, non sports and tennis fans who are Venus and Serena vs Sharapova, more would know who Serena and Venus are, than Sharapova. People who are into models and fashion, would know probably know who Sharapova is more. Yes. But the first group I mentioned is the larger part of the population. Most people are "casual" something, and "non sports fans", as opposed to people into fashion and Glamour. Let's be honest.
We also have to remember, the Williams have chosen not to be in the spotlight, and subsequently reducing their endorsement potential (by reducing their tennis schedules) for the past several years. Nonetheless, these two are relevant publicity wise every time they show up, though they haven't put the work in the past several years like Maria and the other players have (along with the accompanying results). That says a lot, if they did hang around and competed on a high level, what would their fan interest be then?
I think Sharapova has a small (compared to other athletes), but very passionate, dedicated fan base. While more casual fans and non-fans actually know who the Williams sisters are, due to their legacies and impact in the sport.
But hey, people can believe what they want to believe.