What is Calpurnia's relationship to the Finch family in To Kill a Mockingbird?

Nominally, Calpurnia is the loyal housekeeper for Atticus Finch; moreover, she is a mother figure for Jem and Scout. Indeed, as Atticus says, Calpurnia is a member of the Finch family.
All her life Calpurnia has been involved with the Finch family; she grew up, as she tells Scout, "between the Buford Place and the Landin'." Calpurnia adds,"I've spent all my days workin' for the Finches or the Bufords, an' I moved to Maycomb when your daddy and your mamma married." (Ch. 3) After Scout's first day of school, Calpurnia sees how upset the little girl is, so she bakes crackling bread, especially for Scout. Also, she tells Scout, "I missed you today.... The house got so lonesome long about two o'clock I had to turn on the radio."
Calpurnia has been a teacher of Scout's, having taught Scout handwriting. Atticus contends, "She’s a faithful member of this family." (Ch. 14) Calpurnia proves this, as, for instance, when there is a rabid dog on the street, she grabs Jem and Scout by their shoulders and runs them home to phone Atticus.
Calpurnia proves her love and loyalty with regard to the Finch children when she stands up to Lula, a member of her congregation at the First Purchase Church. Lula asks, "I wants to know why you bringin' white chillun to n****r church." Calpurnia replies, "They's my comp'ny." (Ch. 12)

Calpurnia is the Finch's African American cook who also looks after the children while Atticus is at work. In Chapter 12, Calpurnia explains to Scout and Jem that she grew up between the Buford Place and Finch Landing where she worked for Atticus's father. Calpurnia then moved to Maycomb when Atticus got married and has worked in his home ever since. Calpurnia is essentially family and Atticus values her skill set and personality. He defends Calpurnia in front of his prejudiced sister and explains how important Calpurnia is to their family. Although Calpurnia is not afraid to discipline Scout and Jem, she is also sympathetic to their needs. She not only teaches Scout how to write but also keeps her company when Jem and Dill play together. Calpurnia teaches Scout the importance of respecting others and also introduces the children to the African American community by inviting them to First Purchase African M.E. for Sunday service.