。
She's no less beautiful than her sister.
Literal
She [topic-は] older-sister [target-に] not-being-inferior beautiful.
に劣らず is a fixed comparative phrase meaning 'not inferior to, no less than, on par with.' Built from 劣る ('to be inferior, be lacking') + the negative-adverbial ~ず. The に marks the standard of comparison: 姉に劣らず ('no less than her sister'). A literary, slightly elevated frame for equal comparison. 姉 specifies an older sister — Japanese family terms encode birth-order distinctions: 姉 (older sister) vs. 妹 (younger sister), 兄 (older brother) vs. 弟 (younger brother) — without the gender-neutral 'sibling' option in everyday speech.