Dom no Otouto wa Himitsudarake

Chinami and Shuugo became step-siblings through their parents' remarriage. Their unusually close bond often leads others to say they have a brother complex. After finding out that his older brother is a Sub, Shuugo became incredibly overprotective, which led Chinami to decide to find a partner so he wouldn't be a burden anymore. However, during the partner-finding process, Chinami is suddenly given an order from someone he just met. Somehow, Shuugo, knowing where his brother is and who is supposed to be Normal, saves Chinami from the Dom. Being given soothing orders to calm down, Chinami realizes that Shuugo's commands aren't like the other Dom's, and in fact, they're arousing. After their encounter, Chinami just can't seem to stop reacting to his brother's voice... An affectionate, obsessive younger brother and a kind-hearted, innocent older brother— stepbrothers in a Dom/Sub universe!!

(Source: Houbunsha, translated)