Giri no Kyoudai dakedo Koibito no Futari wa Doudou to Tsukiaitai

Kazuma and Yuuya are stepbrothers who are actually lovers and live under one roof (in their parents' house), secretly doing the things lovers do under their parents' radar. A particular close call is intercepted by Kazuma's quick-thinking. While Yuuya is determined to keep their relationship under wraps, he also begins to feel anxious and conflicted for how long they can keep it a secret.

On the other hand, Kazuma meets Yuuya's student, Matsubara, from his part-time as a cram school tutor by chance when they're out shopping. Seeing their closeness, Kazuma's instincts tells him that he should keep an eye out for him. He is also curious how Yuuya is doing at the cram school, with Yuuya having a charm that attracts those younger than him...

(Source: Takeshobo, translated)

Note: Includes an extra chapter.