Commentators explain that the 'brotherly covenant' refers to the historic alliance of friendship and trade between King Hiram of Tyre and Israel's kings, David and Solomon. This wasn't just a political treaty; it was built on mutual respect. Tyre's sin was especially grievous because they betrayed this long-standing friendship by selling Israelites into slavery to their enemies, the Edomites, for profit. God holds nations accountable for breaking solemn agreements.