Chizuko's aunt is actually the wife of Shoji Mikamo, the younger brother of Chiko's father, so she and Chiko are not related by blood. She is responsible for slowly poisoning Chiko with food containing lead in order to take control of Chiko's fortune. While her husband initially supported her plan, she poisons him while Chiko is with Nijū Mensō's group and he dies of lead poisoning. When Chiko returns, she continues to try to eliminate her niece in various ways, though Chiko always manages to elude her. When Tome learns the truth behind her actions, Tome successfully blackmails her into giving Tome full guardianship of Chiko and a mansion for them to live in. Chiko's aunt gives up trying to kill her niece after Akine threatens to reveal the truth to the media following a failed assassination attempt on Chiko; when she asks if Chiko still hates her, Akine replies only that Chiko likely pities her aunt. Yoshie is aided by a lawyer named Kurosaki in her attempt to take Chiko's fortune.  (Source: Wikipedia)

Chiko is the daughter of the wealthy Mikamo family who has to live with her aunt and uncle after her parents passed away. Because her aunt wants to inherit the Mikamo family's wealth, she gives Chiko poisoned food. One day, however, she's kidnapped by the Nijuu Mensou (20 Faces) and decides to join his clan. <br><br>
(Source: ZOMG)

The story revolves a female thief named Chico, who is the spiritual successor to the Kaijin Nij&#363;-Mens&#333; (The Fiend with 20 Faces) thief character created by famed suspense and detective novelist Ranpo Edogawa.<br><br>
(Summary from ANN's entry for the anime based off of the manga)

Yoshie Mikamo