Kyoudai no Rule.

Rule 1, treat each other like family. Rule 2, set a time limit. Rule 3, don't cross the final line... These are our rules as brothers. High school senior Mikage Aiba has one younger step brother, Tomoki. While the elder sibling prefers rules and order, the younger is more easy-going and loose. Despite their two opposing personalities, the two have been close ever since their parents got remarried 12 years prior. However, when Mikage gets accepted into university, the feelings they can no longer push down shatter their "order," and a new set of rules are laid down between the two... (Source: Media Do) Note: Includes one extra chapter.