A Brief History of the Drikung Kagyu"A system of two lineage holders was established, that of the elder (Chetsang) and the younger (Chungtsang) brother." They aren't considered brothers by blood, but by lineage. Chetsang Rinpoche is considered an emanation of Chenrezig.
ergruber.com
Gruber, Elmar R. (2010). From the Heart of Tibet: The Biography of Drikung Chetsang Rinpoche, the Holder of the Drikung Kagyu Lineage. Boston: Shambhala. ISBN978-1-59030-765-6. Synopsis