。
She's not as pretty as her sister.
Literal
She [topic-は] older-sister [extent of-ほど] not-beautiful.
The basic AはBほど~ない frame — 'A is not as ~ as B' — is a workhorse Japanese comparative, used wherever English would say 'not as X as Y.' Without an emphatic は inserted before ない, the negation reads as a flat, matter-of-fact assertion rather than a pointed contrast. ほど ('extent') marks the standard against which the comparison falls short, paired with 美しい in its negative form 美しくない.