wait who was yuri's unrequited love to him like what is their relationship????

cmiiw. Yuri is in love (or is it just a possessiveness, idk) with his oldest noona. they're not blood related it seems, the noona is adopted. and the teacher (Sunwoo) is in love with noona's husband. Yuri is pissed with the teacher because he thought that Sunwoo also loves noona.