Ani no Senaka

Summary

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 those 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 sisters death, the dangerous balance between them finally starts to crumble. (Source: Moi-x Ryu Scanlations)