The character introduced in Rugrats in Paris who becomes Chuckie’s stepsister is Kimi Finster.
Kimi appears in the 2000 film Rugrats in Paris: The Movie. She is the daughter of Kira Watanabe, who meets and marries Chuckie’s father, Chas Finster. That marriage makes Kimi Chuckie’s stepsister and gives him the new family relationship he has been seeking.
The film centers on Chuckie’s wish for a mother after the death of his birth mother, Melinda Finster. In Paris, Chas becomes involved with the scheming Coco LaBouche, but the Rugrats eventually help expose Coco’s motives. Kira proves to be caring and supportive, and she becomes Chuckie’s stepmother as well as Kimi’s mother.
A related detail sometimes causes confusion: Kimi is initially Chuckie’s stepsister, but later franchise material treats her as his adopted sister. Angelica, Susie, and Dil are already established characters from other parts of the Rugrats story and are not the new stepsister introduced in this film.