Why does Jem say that Scout is getting more like a girl?
In Chapter 6, Jem and Dill plan their nighttime raid on the Radley yard, and Scout opposes their plan out of fear that Boo Radley will track them down and kill them. After the boys explain to Scout that they simply plan on sneaking into the Radley yard and peeking through their windows to get a look at Boo, she attempts to dissuade them. Jem responds by telling Scout,
"Scout, I’m tellin‘ you for the last time, shut your trap or go home—I declare to the Lord you’re gettin’ more like a girl every day!" (Lee, 53).
Jem's comment insults Scout, who is a tomboy and prides herself on hanging out with her older brother and Dill. Jem knows that Scout is embarrassed to be referred to as a "girl," which is synonymous with the word sissy, and so she will stop complaining about their risky plan. After Jem accuses Scout of acting like a girl, Scout mentions that she has no other choice than to join the boys on their nighttime raid. The children then proceed to sneak into the Radley yard and barely escape after Nathan Radley runs out onto his back porch wielding a shotgun.
Jem is noticing that Scout is becoming more and more hesitant in doing things that involves Boo Radley. I think you might be refering to the part in the novel where the kids are in the Radley garden. As soon as Scout pipes up about her fears, Jem says that Scout is acting like a girl. He sees Scout as nagging him into doing the right thing rather supporting him. Also Dill is with the Finch kids during this episode. Dill does not show any fear in being on the Radley property and he is close in Scout's age. The only reason, Jem reasons, that Scout doesn't want to see Boo is the fact she is a girl.
Jem knows Scout well. He understands that she sees being a girl as a negative thing. Scout looks around and, with the exception of Miss Maudie, sees women as being weak and restricted by society. She doesn't want to wear dresses and go to tea. She wants to play, like the boys do. Jem is clever older brother -- he knows the best way to convince Scout to do what he wants is to accuse her of being a girl. It's a dare.