。
She's a beauty no less than her sister, if not more so.
Literal
She [topic-は] older-sister [target-に] surpassing [even if-とも] not-being-inferior beauty is.
A fixed and slightly literary phrase: ~に優るとも劣らぬ ('not inferior to, perhaps even surpassing'). The ~とも here is the concessive 'even if' — even if not surpassing, certainly not inferior. The whole frame is a polite way to assert equality with a hint of 'maybe better,' useful in formal compliments and evaluative writing. 劣らぬ uses the classical negative ~ぬ.