Why does Alexandra think Atticus should dismiss Calpurnia?
While it is not uncommon for Southern families to have a black maid, Aunt Alexandra apparently feels that Calpurnia exerts too much influence on her brother's children in areas that have nothing to do with her occupation.
One evening shortly after Aunt Alexandra's arrival, Scout asks her father what the word "rape" means. Atticus replies that it is "carnal knowledge of a female by force." This definition is one that little Scout probably does not understand, but it satisfies her. So she then asks her father why "comin' from church that day" Calpurnia told her to ask her father. When Atticus asks Scout to explain, she tells her father that they went to church with Calpurnia the Sunday that he was in Montgomery, and on the way home she asked Calpurnia about rape. Startled to hear that her niece and nephew went to a church with Calpurnia, Aunt Alexandra drops her embroidery into her lap and just stares at the children.
Then, because Scout does not understand her aunt's reaction, she innocently asks her father if she and Jem may join Calpurnia next Sunday. Alexandra interjects, "You may not." Scout disrespectfully tells her, "I didn't ask you!" Of course, Atticus makes Scout apologize, and Scout leaves the room. She overhears Alexandra tell Atticus:
"You've got to do something about her....You've let things go on too long, Atticus, too long."
It is then that Alexandra suggests that Calpurnia is no longer needed, but Atticus insists that she is a "faithful member" of their family. Clearly Alexandra does not agree as she seems to believe that Calpurnia has too much influence on the children, and she is not a suitable influence, at that.
The reason she says to get rid of Calpurnia is that she isn't needed any longer. However, Alexandra suggests getting rid of Calpurnia after Scout back talks her (Alexandra), and after there is clearly a disagreement about who is in charge and which rules Scout is to obey, so one can assume there's a power struggle, and a conflict over order.
Alexandra is dissapointed in Scout and appears to want to take her into her own hands and recover her; "You have let her go to far,". I believe that she sees Calpurnia as a bad, Negro influence on her and wants to remover her from the picture. She sees her as a block in the road to her power in the household.