。
She's not as pretty as her older sister.
Literal
She [topic-は] older-sister [extent of-ほど] not-beautiful.
The negative comparative ~ほど~ない ('A is not as ~ as B') closes off 'reaching the extent of the standard.' Here 姉さん rather than the bare 姉: the さん suffix is common when referring to one's own older sister in family-internal or familiar contexts. お姉さん (with the お prefix) is the polite alternative, used either to refer to others' older sisters or when speaking with elevated politeness about one's own. 美しい is the i-adjective for 'beautiful'; the negative 美しくない uses the standard ~くない pattern.