。
She's not as beautiful as her sister.
Literal
She [topic-は] older-sister [extent of-ほど] beautiful [contrast-は] not.
The negative comparative pattern AはBほど~ない ('A is not as ~ as B'). ほど marks the extent of comparison; the negation closes off 'reaching that extent.' The は inserted after 美しく adds emphasis or contrast — '*beautiful she is not* (compared to her sister).' Removing the は gives a flatter assertion; keeping it foregrounds the specific quality being denied. Both readings sit firmly in everyday speech, with the は-emphasized version carrying a slight bite of pointed comparison.