Ane no Shinyuu, Watashi no Koibito.  

My Sister's Best Friend, My Lover., 姉の親友、私の恋人。

Sena is a writer who harbors romantic sentiments towards her sibling, Yuuna. After discovering that Yuuna intends to tie the knot, Kiku, Yuuna's closest companion and ex-partner, requests that Sena become his lover.