Boy/man not family related adult

In this world, there are some people who you are not allowed to fall in love with—— Yuki lives together with his deceased older sister’s husband—Keiichirou. He fell in love at first sight, and continued to hold that feelings secretly since then. But that means nothing, for this love is not to be allowed. One night, Keiichirou hugged Yuki and said “Stay with me”! Since his sister’s death, the dangerous balance between them finally starts to crumble!?

A mysterious man of outstanding beauty appeared before me... just as I lost my father to the hands of death. Although I thought he was a debt collector, I was surprised when I found out he was actually my possible foster father. Before I knew it I already fell for him - Sena-san. One touch from this man makes my heart race! Also, what is the meaning of that sudden kiss?!