What does Aunt Alexandra call Atticus, which Scout thinks is strange in To Kill a Mockingbird?

Scout is surprised to hear Alexandra call Atticus “Brother.”

Scout does not get along with her Aunt Alexandra, who is her father’s sister. She feels her aunt is harsh and exacting. She also thinks Alexandra is unsupportive of Atticus’s defending Tom Robinson because Scout’s cousin Francis repreated some racist comments Alexandra made.

At Christmastime, Scout and her family are visiting Finches’ Landing when her deplorable cousin Frances begins teasing her about her father taking Tom Robinson’s case.  It is an unpopular case in Maycomb because Robinson is a black man accused of raping a white woman. Frances’s comments about what Alexandra said are harsh and dripping with racism:

If Uncle Atticus lets you run around with stray dogs, that’s his own business, like Grandma says, so it ain’t your fault. I guess it ain’t your fault if Uncle Atticus is a nigger-lover besides, but I’m here to tell you it certainly does mortify the rest of the family (Chapter 9).

This angers Scout, and she gets into a fight with Frances. Her Uncle Jack, Atticus’s brother, spanks her for it. She later tells her uncle that Frances “provocated” her, but admits she doesn’t really understand what he said. She just doesn’t like it.

Scout is surprised when Alexandra shows up as the trial begins, doubting her intentions. Alexandra says she is there to support her brother, but Scout sees her as a trouble maker. When Robinson is convicted, Scout is surprised at Alexandra’s sympathy for Atticus.

“I’m sorry, brother,” she murmured. Having never heard her call Atticus “brother” before, I stole a glance at Jem, but he was not listening. He would look up at Atticus, then down at the floor, and I wondered if he thought Atticus somehow responsible for Tom Robinson’s conviction (Chapter 22).

Scout is surprised because "Brother" used in this way is a term of endearment or affection. It is Alexandra letting her guard down and showing her love for Atticus. It shows the softer side of Alexandra.

The verdict saddens the whole family, particularly Jem. He takes it very hard because he was convinced the verdict would be an acquittal. Alexandra understands Atticus worked hard on the case. When Tom Robinson tries to jump the fence and is killed, Alexandra shows sympathy for Atticus. Clearly, she is not the bitter and heartless woman that Scout thought she was. She may be racist, and she may not approve of Atticus's actions, but she will support her brother and she does care about him.