Israel's treacherous sister, the southern kingdom of Judah, saw what Israel did and what happened to her. However, Judah did not fear but went and committed fornication also (vv. 7-8). Judah saw that her sister was punished for the evil thing she did, but Judah did not learn from this. Rather, she followed Israel to do the same thing.
In spite of all this, the treacherous southern kingdom of Judah did not return to Jehovah with all her heart, but instead falsely (v. 10).
In verse 11 Jehovah said to Jeremiah that Israel the apostate has shown herself to be more righteous than Judah the treacherous. Israel was better than Judah.
Jeremiah 3:12—4:2 covers Israel the wife's return—a loving pleading of Jehovah the Husband. What we have here is not Israel's actual return but the aspiration of Jehovah the Husband for her to return.
Jehovah sent His prophet to call to the north (referring to Assyria, to which Israel was captured) and ask Israel, His apostate wife, to return and acknowledge her iniquity—her transgression against Him in turning to strange gods under every flourishing tree (vv. 12-13).