Tatta Hitotsu No Koi: love story, but main character takes care of his sick brother.
Nobody Knows(movie): About a boy who has to take care of his three younger siblings (one brother, two sisters)
These are considered comedies, but I'm adding them in case anyone else is interested:
Himitsu No Hanazono: four brothers work together to publish a "girly" manga under a pseudonym.
Yamada Taro Monogatari: high school boy taking care of his 6 younger siblings
Atashinchi No Danshi: This one is not about a brother taking care of his siblings, but it is about a group of brothers.
Oh, and I second Ryusei No Kizuna and Brother Beat.