Hegesippus

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.

Certain of these heretics brought accusation against Symeon, the son of Clopas, on the ground that he was a descendant of David and a Christian; and thus he suffered martyrdom, at the age of one hundred and twenty years, while Trajan was emperor and Atticus governor... They came, therefore, and took the lead of every church as witnesses and as relatives of the Lord. And profound peace being established in every church, they remained until the reign of the Emperor Trajan, and until the above-mentioned Symeon, son of Clopas, an uncle of the Lord, was informed against by the heretics, and was himself in like manner accused for the same cause before the governor Atticus. And after being tortured for many days he suffered martyrdom, and all, including even the proconsul, marveled that, at the age of one hundred and twenty years, he could endure so much. And orders were given that he should be crucified.

Clopas was a brother of Joseph.