The letters use the nickname only my dead sister knew

Dear Delyara, For three months, someone has been leaving handwritten notes inside my mailbox. They contain no threats, only details about my childhood and a nickname my sister used before she died. The first note said, “Little Sparrow still checks the window twice.” I do check the window twice. My sister began calling me Sparrow when we were children because I was frightened of birds but drew them constantly. Nobody else used it. The police say the notes are strange but not criminal. I have installed cameras, changed locks, and asked relatives whether my sister ever mentioned the nickname. Everyone says no. Yesterday’s note described a red coat I wore the day she drowned. That detail appears in no photograph. I have begun questioning whether my memory is wrong, whether my sister told someone, whether a relative is playing a cruel game, or whether the writer knew us when we were children and has waited thirty years to become visible. Tonight, there is a new envelope on the floor beneath the mail slot. I have not opened it. I am sitting across the hallway watching it as if paper can breathe. Part of me is terrified of the person outside. Another part wants the notes to contain something only my sister could have known, because grief is vulnerable to any lie that resembles a return.