Cornplanter and Handsome Lake were half brothers, born to the same Seneca mother at Canawaugus and raised in the same Seneca community, and their two lives diverged and rejoined at some of the most consequential moments in Haudenosaunee history. While Cornplanter rose as a war chief and treaty diplomat, Handsome Lake lived for years within the pressures of land loss and the alcoholism that had taken hold of Seneca communities after the Revolution, pressures Cornplanter's own diplomacy had done little to relieve. In June 1799, ill at Cornplanter's own settlement on the Allegheny, Handsome Lake collapsed into a state that appeared to onlookers as death. Cornplanter was called to his brother's side, and it was in that comatose state, with Cornplanter and others gathered around him, that Handsome Lake received the first of the visions that became the Gaiwiio, the good message that founded the Longhouse religion.
Cornplanter's own response to his brother's teaching was not immediate. He had already invited Quaker missionaries onto his land to run schools, a choice the Gaiwiio's call to preserve traditional Seneca ways sat uneasily beside. Only later, after the War of 1812 left him disillusioned with the accommodation he had spent his career pursuing, did Cornplanter close the schools he had welcomed and turn toward his brother's reform, by tradition burning his old military uniform and breaking his sword as a sign of the change. He did not fully sever his ties to the Quakers, but the reversal marked him, by the end of his long life, as a supporter of the very religious revitalization his brother had begun at his own doorstep. Cornplanter died on February 18, 1836, on the Pennsylvania land grant that carried his name.