Despite the close bond between brother and sister, Shawn starts acting abusively towards Tara, both physically and mentally. A prime example comes in chapter 12 of Educated, "Fish Eyes". One night, Shawn demands that Tara bring him a glass of water. When she answers back by asking him if he has a broken leg, Shawn further demands that she apologize for her insolence. When she refuses, Shawn physically assaults her, grabbing Tara by the hair and dragging her off to the bathroom, where he sticks her head down the toilet.
Eventually, Shawn drags Tara back up again, but then he starts to twist her wrist, which, as one can imagine, is very painful. The pain gets so bad that Tara has no choice but to mutter that she's sorry. Having gotten what he wanted, Shawn walks off, leaving Tara alone in the bathroom.
As she looks at herself in the mirror, Tara sees herself crying. But she tells herself that she's only crying because of the pain, not from anything else. This leads her to conclude that she's unbreakable—that no matter how much physical pain is inflicted on her, it won't effect her.
For years, Tara believed this, until one day, she realized that such traumatic events not affecting her was the effect. In other words, the fact that her brother's violence had no effect on her showed the extent to which he'd successfully numbed her emotions through his regular acts of violence and abuse.