they're actually cousins he adopted by seme parents
(source: closed the last door)
the three brothers is adorable
Related Series
1 Close the Last Door - In Saitoh's wedding, brokenhearted Nagai got over by sleeping with bride's guest Honda just to find out the bride ran away! Σ(￣∇￣") Love-Triangle
2 Aiteru Door kara Shitsurei Shimasu yo (Side Story)3 - Focus on the two elder bros of Honda
3 Arienai Futari (Spin-Off)
Into Your Heart Through The Door