Aka-chan to Boku

Takuya is a normal Japanese elementary student whose mother died not very long ago, leaving him alone with his father and his baby brother Minoru. But his father is a busy man, and Takuya has to look after Minoru. Because of this responsibility that would normally never burden a child of his age, Takuya sometimes resents his fate and his baby brother... but his love for Minoru gives him the strength to carry on.
(Source: ANN) Included one-shots:
Volume 2: Time Limit
Volume 4: Secret Boy ni Tee Dasuna!!
Volume 6: Baka de Karukete Wagamama de
Volume 15: Natsu to Kimi no Irradiation