My brother has called upon the words of Zenos, that redemption cometh through the Son of God, and also upon the words of Zenock; and also he has appealed unto Moses, to prove that these things are true.
That’s what Amulek says after Alma talks about Zenos and Zenock. But Alma and Amulek aren’t brothers! Well, they kinda are brothers.
As children of our Heavenly Father, all men and women are spiritually brothers and sisters. In the Church, male members and friends of the Church are often addressed as brothers.
Brethren, Brother – Guide to the Scriptures
Just not temporal brothers.