Eusebius, Church History IV.22 (quoting Hegesippus) — Nicene and Post-Nicene Fathers, series 2, vol. 1 (Arthur Cushman McGiffert, 1890) (public domain)
4. The same author also describes the beginnings of the heresies which arose in his time, in the following words: And after James the Just had suffered martyrdom, as the Lord had also on the same account, Symeon, the son of the Lord's uncle, Clopas, was appointed the next bishop. All proposed him as second bishop because he was a cousin of the Lord.
Therefore, they called the Church a virgin, for it was not yet corrupted by vain discourses.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 1 (Arthur Cushman McGiffert, 1890) (Public domain).
Why Pitre cites it
'Symeon, his cousin, the son of Clopas, was appointed bishop, whom they all proposed because he was another cousin of the Lord.' Hegesippus' disinterested testimony that the Lord's 'brothers' were his cousins.
Notes
§4 of ch. 22 plus the short sentence that follows it ('Therefore, they called the Church a virgin ...'), which New Advent prints as part of the same Hegesippus quotation.