Sweet Regard (2005)

It had already been eight years since he and I first met. My sister’s much younger than me, but he was even younger. "How could I give my precious sister to such a child" I thought. But though I was against it, the two of them married, and then my dearest sister suddenly died... Tormented by the grief of having lost everything, and by hellish sadness, I was saved by he whom I should have hated most. The time we spent together quickly filled the void between us.

Source: www.anisearch.com/manga/1276