Giving exact page numbers is going to be difficult because of the number of different editions of this book that out in publication at this point. Additionally, Aunt Alexandra is mentioned a lot of times in the book. Her name is mentioned more than 100 times throughout the text.
In the text that I have, Aunt Alexandra is first mentioned by name on page 55 in chapter 9. Scout is being goaded by Cecil into a fight, and Scout, for the first time ever, walks away from a fight.
I drew a bead on him, remembered what Atticus had said, then dropped my fists and walked away, "Scout's a cow--ward!" ringing in my ears. It was the first tie I ever walked away from a fight.
She is really proud of herself for doing it, and then she begins telling the reader about Aunt Alexandra, Uncle Jimmy, Francis, and Christmas time with them. Basically, Scout and Jem do not like spending Christmas with that side of the family.
Uncle Jimmy doesn't talk much and Francis is boring and a tattletale.
Talking to Francis gave me the sensation of settling slowly to the bottom of the ocean. He was the most boring child I ever met.
As for Aunt Alexandra, she disapproves of basically everything Scout. She doesn't like how Scout acts or dresses especially. A reader shouldn't think that Aunt Alexandra is a "bad guy" or hates Scout. Aunt Alexandra simply believes that Scout needs to act more like the stereotypical southern lady.
Aunt Alexandra's vision of my deportment involved playing with small stoves, tea sets, and the Add-a-Pearl necklace she gave me when I was born; furthermore, I should be a ray of sunshine in my father's lonely life.
The next time that Aunt Alexandra is featured prominently in a chapter is chapter 13. In my edition, that begins on page 90. In this chapter, Aunt Alexandra arrives at the Finch household and explains to Scout that she is there to stay awhile and help Atticus out with the house and the kids. She specifically mentions that she can bring some "feminine influence" to the house. Needless to say, Scout is not happy. The two characters are frequently at odds with each other, but by the end of the book, Scout and readers have no doubt that Aunt Alexandra is a fierce defender of her family. For example, there is this quote from page 170 that shows her devotion to Atticus despite disagreeing with him.
"I can't say I approve of everything he does, Maudie, but he's my brother, and I just want to know when this will ever end. Her voice rose. "It tears him to pieces. He doesn't show it much, but it tears him to pieces."