Share your bento with your little brother.

Literal

Bento [obj-を] younger-brother [to-に] divide-give (polite command).

分けてあげる is 'share (with someone)' — literally 'divide and give (outward).' The benefactive ~てあげる marks the giving direction. ~なさい closes as a polite but firm parental command. に marks the recipient of the sharing.