The descendants of Israel, North and South, inherited the covenant of Abraham, Isaac and Jacob at Mt. Sinai hundreds of years before. Yet, instead of living in that covenant, the children of Israel preferred to imitate the practices of the nations around and actually surpassed the surrounding nations around in their iniquity. This is why God judges the entire family of Israel much more severely than He punishes the other nations. This is the prophet’s warning in Amos 3.
In Amos 1-2, God used the sins of Israel’s nations as a rebuke against Israel. In Amos 3, He was not just addressing the Northern Tribes but also Judah.