私ト一ツノ心臓ヲ (Watashi to Hitotsu no Shinzou o)

Background
Watashi to Hitotsu no Shinzou o is the second song in Sister's Story, a series by masa.

Immediately after Tsukoyomi's murder, Amaterasu modifies her appearance to match her sister's down to the finest details so as to avoid the repercussions of what she's done. Amaterasu, disguised as her older sister, then lives on for more than ten more years without issue.

Once Amaterasu turns nineteen years old, Tsukuyomi returns as a spirit dressed as Amaterasu's original, true appearance in an attempt to confuse her living sister as to who she truly was all along. Amaterasu had already developed her sister's mannerisms over the years and had begun to question her memory of her true identity, and Tsukuyomi's interference caused her to have serious thoughts about her actually being the older sister all along and her younger sister being the one who died.undefined