Ane no Onaka o Fukuramaseru wa Boku

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.