The Evil Shadow in the Mirror: I Solved the Cold Case for My Deceased Sister

In the third year after my sister committed suicide, I saw the reflection of a strange woman in her dressing mirror - the person in the mirror was wearing my sister's dress, had the same type of strangulation marks on her neck, and was holding a piece of paper with my name on it in her hand. When I followed the clues in the mirror, I discovered that my sister's "suicide" was a faked murder, and all the suspects' confessions pointed to a non-existent "I".