。
She is no less charming than her older sister.
Literal
She [topic-は] her [genitive-の] older-sister [than-に] not-falling-behind charming is.
~に劣らず is a written-style comparative meaning 'no less than, not inferior to.' 劣る ('to be inferior') takes the entity being compared with に, and 劣らず is the negative continuative form (劣ら-ず) used adverbially: 'without falling behind.' The である copula at the end stamps this as written or formal register — a casual speaker would use だ or rephrase as ~と同じくらい~. Note the rhetorical inversion: rather than saying 'as charming as her sister,' Japanese builds the comparison from 'not inferior to her sister' — a slightly stronger statement.