Long ass name: [HalftoneDot] The Shrine Maiden of Hakucho Shrine Saves Her Sister Through Forbidden Feelings [Director Cut]
https://www.dlsite.com/maniax/work/=/product_id/RJ01515297.html

Momiji has secretly loved Ren, her sister's boyfriend, for many years. When a Death God curses her sister, Momiji offers her spiritual power to save her.
The method—intimate connection. But each time they connect, Momiji fades away. Her voice disappears. Her hands turn transparent. Even her name is forgotten.
Still, she keeps fighting. "Please don't forget me, Ren."
A forbidden pure love touching SLG about a woman who vanishes for her sister's happiness.​

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