It usually takes an extreme event for a dysfunctional family to repair itself. Maybe there's a huge fight at Thanksgiving, or people reconciling at the funeral of a grandparent—in I Know This Much is True, Thomas Birdsey chops his own hand off. Yeah, that's extreme.
Thomas is Dominick's slightly older twin brother who has paranoid schizophrenia. He cuts his own hand off in a public library (we hope he did it very quietly) as a statement against the United States going to war. He not only thinks he's being pursued by the CIA and Communists alike, but believes he's important enough to stop global conflict by making this sacrifice. He laments, "Sometimes I wonder why I have to be the one to do all this, Dominick. […] Why it's all on my shoulders. It's hard" (1.43). We can only imagine.
While there's no way to pinpoint the exact cause of Thomas's mental illness—is it genetic? Is it the way he was raised? A combination of both?—Dominick spends a lot of time trying to trace the root of Thomas's problems.
When the brothers are growing up, Thomas is always closer to their mother. She dotes on him, has tea parties with him, and generally does everything she can to shelter him. She's not very good at protecting him from their stepfather, Ray, though, who abuses Thomas by taping his hands together and locking him in a cupboard under the stairs. (Who is he? Mr. Dursley?)
Because he's momma's little boy, Thomas is always kind of selfish, but becomes increasingly more self-centered as his disease worsens. When Ma is dying, all Thomas wants is to go to McDonald's, and he pouts until he gets his way. And when she does die, he simply asks if he can have back the collage he gave her for her birthday. Dominick often wishes that, for once, Thomas would just think of someone other than himself.
Thomas spends a lot of time in various institutions, even before chopping his hand off. But after amputating his hand, his illness accelerates. He makes up abuse that never happened, like talking about Ray urinating on his clothes or raping his mother in front of him.
While it's impossible to say whether or not this happened, it's doubtful that it did. But why does Thomas make this stuff up? Is it because of his illness? Or is it because no one really cares about the actual abuse he was subjected to so, in order to get help, he has to make it seem worse? Dominick thinks that a lot of it is that he needs attention and love. "Nobody's ever chosen Thomas. […] Nobody except Ma" (25.203), he says, and that's why Dominick fights to get Thomas out of Hatch, the mental institution he's court ordered into after the hand incident.
The problem is that Dominick is a horrible caretaker. Just like when Ma died, all Thomas wants is a Happy Meal when he gets out of Hatch, but Dominick doesn't want to get him one. "Those things are for little kids" (40.206), he says, hating on his brother instead of helping him.
Dominick manages to get Thomas into a halfway house, but Thomas escapes that night, falls off/jumps into (no one knows for sure) the local waterfall, and dies. "Whether Thomas has jumped or fallen in, none of us could really say" (40.240), Dominick notes, and although he mourns the loss of his brother, Thomas's death also turns out to be a relief for him.
Dominick sometimes says, "There have been times when I've ached to have him back again" (19.150), and therein lies the explanation for why Thomas's death is a relief: Dominick loses his brother to schizophrenia long before he actually dies. Once he's dead, Dominick no longer has to confront this loss constantly, becoming free to bury his brother and move on with his life. It's a pretty big bummer all around, if you ask us.