Scout notices that jem, who has turned 12 at the beginning of ch 12, has changed. describe how jem is different. how has the relationship between the siblings changed?

Once he became 12 years old, Jem began to ditch his childish ways , even at times scolding Scout for disturbing his life. This change would lead him to be more interested in the concerns of the adults, spending more time alone and away from Scout.

The relationship of the siblings changed drastically and so suddenly that Scout did not seem to understand it. She finds it unfair and unacceptable that Jem is behaving so different in such a short time. She wished that someone would "beat him up" for the way he's acting with her and telling her to mind her own business and leave him alone.  

Explanation:

In Chapter 12 of Harper Lee's To Kill A Mockingbird, the narrator "Scout" directly states that "Jem was twelve", meaning he's grown up. She also mentions that "He was difficult to live with, inconsistent, moody. His appetite was appalling, and he told me so many times to stop pestering him" which seemed to greatly disturb or rather annoy her.

This change in Jem, which Calpurnia termed as him becoming "Mister Jem", changed the siblings' relationship for Jem began to think less about their childish games and more into the 'adult' life. Calpurnia also told Scout that he's "gonna want to be off to himself a lot now, doin’ whatever boys do", thus leaving Scout to entertain herself all alone without Dill. This hurt Jem for she had always had Jem as a play partner and in everything they do. Now, she's left alone to play in her own childish world while Jem begins to act and get interested more into the adult world. But for Scout, this change is something that she finds unacceptable, for Jem "acquired an alien set of values and was trying to impose them on me". This change in their relationship led Scout to spend her summers more with Calpurnia in the kitchen.  

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