。
She works just as hard as her brothers do.
Literal
She [topic-は] brothers [with-と] same-extent earnest [adverbially-に] works.
The equal-comparison frame XはYと同じくらい~ ('X is as ~ as Y') anchors the standard of comparison with と and locks the degree to 同じくらい ('the same extent'). 一生懸命 is one of those Japanese fixed phrases that transcends register — five-year-olds and CEOs both use it for 'wholeheartedly, with all one's might.' The literal 'one-life-hang-fate' captures its origin: putting one's life on the line, derived from the older 一所懸命 ('hanging one's fate on a single place,' originally about samurai defending a single domain).