In A Lie of the Mind, Sally is one of the more minor characters (albeit still a quite important one). Sally is Jake's little sister who continually refuses to be around her big brother. In fact, she doesn't even want to live with him in the same house. Her feelings are so very strong that Sally leaves the moment that Frankie accompanies Jake to the farm.
Sally's big issue is that Jake doesn't take responsibility for himself or for others. A very mature character for her age, Sally doesn't want to have any part in Jake's idea to wheedle out of the trouble he gets himself into (especially for having given Beth a beating for which she suffers severe brain damage). Much of Sally's feelings stem from guilt because she guarded her older brother from the blame in dad's death. In fact, the story begins with Sally talking about finding dad in a broken-down trailer.
Jake always wants to challenge his dad, and proposes a bet that the first person to reach the United States from Mexico would win. It is Sally that puts the true meaning of the race into perspective when she says that "[Jake] had decided to kill him." And, truly, the race ends with Jake's father violently killed and "all over the road like some lost piece of livestock.”
Still, Sally protects Jake from blame and feels lots of guilt. Because of this, however, she no longer wants to guard Jake from further harm. Sally now feels that it is Jake's job to take responsibility for his own erroneous actions. The title, of course, is about the lies people tell themselves (and tell others) to justify their actions in this world. Note the following important quote:
These things—in my head—lie to me. Everything lies. Tells me a story. Everything in me lies. But you. You stay. . . . You are true. I love you more than this life. You stay. You stay with him. He’s my brother.