As Atticus and the children return home in Chapter 22, Aunt Alexandra is waiting up, and, although she is in her dressing gown, Scout notes that she appears to be yet wearing her corset. This particular detail indicates that Aunt Alexandra does not wish to be relaxed, for she feels that she must maintain her composure and strength for Atticus's arrival.
'I'm sorry, brother,' she murmured. Having never heard her call Atticus 'brother,' I stole a look at Jem...
In this remark, Aunt Alexander demonstrates a familial loyalty to Atticus. This emotion is certainly not out of character inlight of her previous discussions about the Finch family tree. For, family loyalties supercede other feelings. Continuing in her solicitous manner, Aunt Alexandra responds to Atticus's comment about the prejudice of the jury being just as much a part of Maycomb County as the missionary teas:
'You are the last person I thought would turn bitter over this.'
Clearly, the sister loves her brother and is concerned about the emotional effect of the Tom Robinson trial upon her brother Atticus.
When Atticus, Jem, and Scout arrive home after the trial, Aunt Alexandra is up and waiting for them in her dressing gown. She first tells Atticus, "I'm sorry, brother," and, as Scout rarely hears her aunt using the word "brother," the reader can infer that Aunt Alexandra feels some sympathy for Atticus.
Aunt Alexandra then asks how Jem is doing, as he is clearly upset about the verdict in the Tom Robinson trial. She says, "I didn’t think it wise in the first place to let them—” but Atticus cuts her off and says that it is right for Jem and Scout to have seen the trial, as Maycomb is their home too. He says that as adults have made Maycomb what it is, the children have the right to see it. Aunt Alexandra then takes issue with what she calls Atticus's "bitterness" over the trial.
From her response, we know that Aunt Alexandra seeks to protect the children from the realities of life in Maycomb. She would rather pretend that the local people are fair minded and benevolent and, for example, contribute to the missionary society. She doesn't want to speak about or have the children witness the reality of the racism and unfairness that surround them.