What does bien comment est tu aussi belle que sa mean in English?

I'm not sure what you're asking, here.

The use of the possessive pronoun "sa" (her) is grammatically incorrect in this context. You must use the personal pronoun "elle" (she).

If you say: "Bien comment est-tu aussi belle qu'elle (?)", you are asking: "Well, how are you as beautiful as she?" - Many females may be offended by this question.

If you want to say: "Bien, tu est aussi belle qu'elle.", it means: "Well, you are as beautiful as she."

I hope this helps.