。
She helped her younger brother with his homework.
Literal
She [topic-は] younger-brother [possessive-の] homework [object-を] helped [for him-てやった].
~てやる is a benefactive auxiliary meaning 'do something for someone' — used when the beneficiary is of equal or lower status, or when the speaker is being casual about their own generosity. 手伝ってやった = 'helped (him) as a favor.' In modern usage, ~てやる can sound condescending or rough, so ~てあげる is often preferred in polite contexts.