What happened to Eli McCullough's brother in The Son?

In Philipp Meyer's novel "The Son," Eli McCullough's brother, who is named Peter, is tragically killed by a group of Comanche warriors during a raid on their family's ranch. This event profoundly impacts Eli, shaping his views on survival and violence as he navigates the complexities of life in the American West. Peter's death serves as a catalyst for Eli's transformation throughout the story.