。
She is no less attractive than her older sister.
Literal
She [topic-は] older-sister [target-に] not-being-inferior attractive [literary copula-である].
A formally polished statement built from three writing-flavor pieces: ~ず (the classical negative-adverbial, equivalent to ~ないで), である (the writing-style copula), and 魅力的 (with the productive ~的 suffix). The frame ~に劣らず ('no less than') sets up the equal-comparison against 姉; である closes it off in elevated register. Common in biographical or evaluative writing where the spoken だ would feel too plain.