Commentaries
25:16-18 We read not that any Midianites died of the plague; God punished them with the sword of an enemy, not with the rod of a father. We must set ourselves against whatever is an occasion of sin to us, Mt 5:29,30. Whatever draws us to sin, should be a vexation to us, as a thorn in the flesh. And none will be more surely and severely punished than those who, after Satan's example, and with his subtlety, tempt others to sin.
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.