Her treacherous sister Judith - These words are a sort of refrain, thrice Jeremiah 3:7-8, Jeremiah 3:10 repeated before God finally pronounces Judah more culpable than Israel.
Matthew Henry
Concise Bible Commentary
If we mark the crimes of those who break off from a religious profession, and the consequences, we see abundant reason to shun evil ways. It is dreadful to be proved more criminal than those who have actually perished in their sins; yet it will be small comfort in everlasting punishment, for them to know that others were viler than they.
SDA Bible Commentary (limited)
Volume 1-7
10. Whole heart. The reformation under Josiah (v.6) was a mere outward turning to God. At heart the people still clung to their idols. After the king’s death they lapsed into open idolatry (2 Kings 23:31, 32; 2 Chron. 36:5-8).
Feignedly. Literally, “in falsehood,” or “in deception.” Judah acted a lie in her pretended reformation.