。
She doesn't study as hard as her older sister.
Literal
She [topic-は] older-sister [extent of-ほど] all-out doesn't-study.
一生懸命 ('with all one's might, wholeheartedly, desperately') is a four-character compound and one of the most common Japanese adverbs of effort. Historically 一所懸命 ('defending one's one fief'), referring to a samurai's plot of land worth defending to the death; the form drifted to 一生懸命 ('staking one's whole life'). Used freely as adverb or na-adjective, it captures everything from intense study to relentless practice. Combined with the negative comparative ~ほど~ない, the sentence draws a contrast: 'not as hard as.'