Eusebius, Church History III.11 (quoting Hegesippus) — Nicene and Post-Nicene Fathers, series 2, vol. 1 (Arthur Cushman McGiffert, 1890) (public domain)
After the martyrdom of James and the conquest of Jerusalem which immediately followed, it is said that those of the apostles and disciples of the Lord that were still living came together from all directions with those that were related to the Lord according to the flesh (for the majority of them also were still alive) to take counsel as to who was worthy to succeed James.
They all with one consent pronounced Symeon, the son of Clopas, of whom the Gospel also makes mention; to be worthy of the episcopal throne of that parish. He was a cousin, as they say, of the Saviour. For Hegesippus records that Clopas was a brother of Joseph.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 1 (Arthur Cushman McGiffert, 1890) (Public domain).
Why Pitre cites it
After James's martyrdom, Symeon son of Clopas was chosen bishop of Jerusalem, 'for Hegesippus records that Clopas was a brother of Joseph.' The report that the 'brothers' were Jesus' cousins, stated as plain history.
Notes
Whole chapter (two numbered sections).