For they assailed you deceitfully when they seduced you in the matter of Peor and their sister Cozbi, the daughter of the Midianite leader, the woman who was killed on the day the plague came because of Peor.”
God had punished the Israelites for their sin with a plague; as a Father he corrected his own children with a rod. But we read not that any of the Midianites died of the plague; God took another course with them, and punished them with the sword of an enemy, not with the rod of a father. 1.
Commenting on Numbers 25:16-18
they vex you with their wiles--Instead of open war, they plot insidious ways of accomplishing your ruin by idolatry and corruption. their sister--their countrywoman. Next: Numbers Chapter 26
For they vex you with their wiles, wherewith they have beguiled you in the matter of Peor, and in the matter of Cozbi, the daughter of a prince of Midian, their sister, which was slain in the day of the plague for Peor's sake. (h) Causing you to commit both corporal and spiritual fornication by Balaam's counsel, .