Her English is just as good as — if not better than — her older brother's.
Literal
She [topic-は] older-brother [to-に] surpassing [even-とも] inferior-not extent English [subject-が] skilled [copula-だ].
勝るとも劣らぬ (まさるともおとらぬ, 'rivaling if not surpassing') is a classical set phrase. くらい ('to the extent of') softens the comparison slightly. 英語が上手だ is the basic 'is good at English' pattern. The sentence uses a formal literary comparison structure (勝る + とも + 劣らぬ, with the classical negative ぬ) in what is otherwise a casual observation.