How does Jing- mei get away with her mistakes?

I don't think that Jing- men really "got away" with making mistakes. She merely rebelled so hard all the time that her mother became used to being disappointed in her.

Check this out on the GradeSaver site,

Even after the fiasco, Suyuan expected Jing-mei to practice piano. Jing-mei flatly refused, saying she would never be someone she wasn't. Suyuan dragged her to the piano and told her, "Only two kinds of daughters ... Those who are obedient and those who follow their own mind! Only one kind of daughter can live in this house. Obedient daughter!" Jing-mei shouted back, "Then I wish I wasn't your daughter. I wish you weren't my mother ... I wish I'd never been born! ... I wish I were dead! Like them" (referring to the babies Suyuan abandoned in China). As though Jing-mei had said the magic words to destroy her mother, Suyuan left the room meekly--and Jing-mei stopped her piano lessons.

From then on, Jing-mei "asserted ... [her] right to fall short of expectations," disappointing her mother time and again.