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.
They vex you with their wiles. Instead of open war, they plot insidious ways of accomplishing your ruin by idolatry and corruption.
Their sister - i:e., their country-woman.