Oshi No Ko

Goro Amamiya is an obstetrician-gynecologist in a rural Japanese town tasked in assisting idol singer Ai Hoshino, who has taken a secret hiatus from her role in the idol group B-Komachi after becoming pregnant with twins. On the night of Ai's delivery, Goro is murdered by a deranged fan of Ai and is reincarnated as Ai's son Aquamarine. In addition, Aqua's twin sister Ruby is a reincarnation of Sarina Tendoji, one of Goro's patients and a fan of Ai. Four years later, Ai is murdered by the deranged fan before he commits suicide, and Aqua deduces that it was their biological father who planned the murder. As the twins reach their teenage years, Ruby follows Ai's footsteps and becomes an idol while Aqua continues to find their father and avenge their mother.

Synopsis from Anime News Network