Great question. So the covenant of brotherhood was an agreement with terms between King Hiram of Tyre, David and Solomon.
"And the LORD gave Solomon wisdom, as he promised him: and there was peace between Hiram and Solomon; and they two made a league together." 1 Kings 5:12
And the LORD gave Solomon wisdom, as he promised him: and there was peace between Hiram and Solomon; and they two made a league together.
-- 1 Kings 5:12 KJV
See also 1 Kings 5:1-12
Joel 3:3-8 fills in the details of this Hebrew trade, where Hiram broke the brotherly covenant and began selling captive Jews to the Grecians.