What makes you think you have to let her know?

I realize you're in the big brother department and would like to protect her. But allow me to ask: would she be any less in need of your protection if she were plain-looking? or homely?

Children are incredibly bright. If she's pretty, she already knows. And I'd be willing to wager that she's already leveraging that knowledge when it suits her.

If you try to prepare her for it, all you're really doing is letting her know how important you think it is. And if she looks up to you, that will reflect on how she values it as a part of her identity. Which I feel is far more risky than calling her 'smart'. Because 'pretty' is transient.