Ane no Onaka o Fukuramaseru wa Boku姉のおなかをふくらませるのは僕 ; 누나의 배를 부르게 하는 건 나 ; Ane no Onaka wo Fukuramaseru wa Boku ; It is me to fill the stomach of my sister

From MangaHelpers: This is the story of a pair of step-siblings who are now living alone together. The high-school-age older sister, Kyoko, is pretty wild (and is even into pro-wrestling), while the elementary-school-age little brother is a fairly serious kid. He is often embarrassed by her teasing. The two of them pass their daily life together, every day cooking for one another.