Nana, an apathetic 24-year-old with nothing much in her life, suffers a car accident and wakes up in her 14-year-old junior high body. As she lives through each day, she starts to remember things she buried deep in her memory, and she can now not remember being involved with her older sister, whom she deeply admires, and her sister's first love, a senior at her school.

From Half-Baked Scanlations:

Ririko, a stylish Charisma Gal from Shibuya, finds herself playing amateur detective after an unexpected turn of events. She is keen to solve cases to prove herself worthy of elite police detective Nii, a man she adores and regards as her fiancé, but her enthusiasm leads her into a whole lot of trouble.

From Manga Art:

Youko spends her days as the primary target of her demonic sister. But how is it that her sister's evil deeds manage to bring happiness to those around her!? Is her sister really an angel, an ally of Justice?