Komentarz do Amosa 1:9

and did not remember the brotherly covenant—which existed between Hiram and Solomon, as it is said: (I Kings 5:26) “And they both made a league together.” And they called each other brothers, as it is said: (ibid. 9:13) “And he said: ‘What are these cities that you have given me, my brother?’”

1:9 And the sin of Tzur is mentioned here when it says they made Israel have the complete exile meaning they sent the refugee israelites who were living peacefully amongst them to Edom. And they didn't remember the convenant that there was between Israel and Tzur. That was in the days of Chiram and Shlomo Hamelech like it says (Malachim 1 5:26) they made a covenant of peace. And they called each other brothers as you see from the pesukim "what are these cities you have given to me my brother?"