Walk through Amos, verse by verse, with the great Bible Commentaries of Matthew Henry (1662 - 1714) & Adam Clarke (1760 - 1832)
1:9
Thus saith the LORD; For three transgressions of Tyrus, and for four, I will not turn away the punishment thereof; because they delivered up the whole captivity to Edom, and remembered not the brotherly covenant: KJV
{The Lord God saith these things, On three great trespasses of Tyre, and on four, I shall not convert it, for they closed together perfect captivity in Idumea, and had not mind on the bond of peace of brethren.} Wycliffe's Bible
The peculiar sin of Tyre is delivering up the whole captivity to Edom, that is, selling to the Edomites those of Israel that fled to them for shelter, or in any way fell into their hands; not caring what hardships they put upon them, so that they could but make gain of them to themselves. Herein they forgot the brotherly covenant, the league that was between Solomon and Hiram king of Tyre
The Tyrians, in exercising cruelties upon these, did it, in effect, on the Jews, with whom they were connected by the most intimate ties of kindred; the two people having descended from the two brothers, Jacob and Esau.