The table of the chancellors' family trees in the New Book of Tang, vol. 72.1, actually listed another older brother and gave his name as Du Fengju (杜鳳舉), but as it would have been highly unusual for father and son to share one character in their name by that point due to naming taboo, and particularly because Du Pengju's and Du Fengju's names were parallel in form to each other, suggesting that they were brothers rather than father and son, Du Fengju was probably an uncle, not a brother, of Du Hongju.
