Why does Jem ask Scout not to do anything to antagonize Aunt Alexandra? (This is referencing the time when Scout asked what "rape" was.)

Chapter 14 of Harper Lee's To Kill a Mockingbird starts out with Scout hearing someone say obscurely, "They c'n go loose and rape up the countryside for all of 'em who run this county care" (135). She remembers that she had asked Calpurnia what the word rape means, but Cal told her to ask her father. So when she hears this comment, she decides to ask him. Atticus doesn't seem to catch on to what...

Chapter 14 of Harper Lee's To Kill a Mockingbird starts out with Scout hearing someone say obscurely, "They c'n go loose and rape up the countryside for all of 'em who run this county care" (135). She remembers that she had asked Calpurnia what the word rape means, but Cal told her to ask her father. So when she hears this comment, she decides to ask him. Atticus doesn't seem to catch on to what Scout is saying and has her explain as Aunt Alexandra listens in. Before Scout can get an answer, Aunt Alexandra shifts the dramatic focus off of the question at hand to the apparently unfathomable fact that the kids visited a colored church with Calpurnia. 

Then Scout forgets about her first question about rape and mentions that Calpurnia had also invited the kids to visit her home sometime. Scout asks Atticus if that would be possible and Aunt Alexandra says, "You may not" (136). Scout says, "I didn't ask you" and Atticus makes her apologize to her aunt. The adults get into an argument and Scout goes off to the bathroom to regroup. This is where Jem gets involved and asks her, "Scout, try not to antagonize Aunty, here?" (137).

Jem explains that the trial is weighing heavily on Atticus's mind, but that's all he can really explain from his twelve year-old perspective. What he means to tell her is Atticus doesn't need Scout to get Aunt Alexandra all screaming and arguing with him about the way the kids are being raised because he's bearing the weight of the community on his shoulders as it is with the Tom Robinson case. Rather than articulate this properly, though, Jem tells Scout that he'll spank her if she antagonizes Aunty again. Scout never gets the exact definition of rape explained to her, either. She partly figures it out later while watching the trial.