Before Henry is told that his sister, Louisa, is responsible for killing his brother, Franklin, he comes to the conclusion himself at the end of chapter 20.
And suddenly, Henry knew everything about the accident, knew it with the sureness and precision as sharp as a geometric axiom.
Louisa is the American girl Chay is seeing and was in the car with him when Franklin was hit. However, as he states on page 277, she wasn't just in the car—she was driving the car when she saw Franklin running towards them. Knowing what Franklin would think, and perhaps do, if he saw her with a Cambodian man, Henry imagines that she panicked, lost control of the car, and hit her own brother. Chay only tried to protect her.
In chapter 23, page 279, Louisa sits down on Henry's bed and says, "I have something to tell you." The author does not go into any detail about what is said.