And Aaron said unto Moses, Alas, my lord, I beseech thee, lay not the sin upon us, wherein we have done foolishly, and wherein we have sinned.

Wesley's Notes

12:11 Lay not the sin - Let not the guilt and punishment of this sin rest upon us, upon her in this kind, upon me in any other kind, but pray to God for the pardon and removal of it.

Jamieson-Fausset-Brown Bible Commentary

11-13. On the humble and penitential submission of Aaron, Moses interceded for both the offenders, especially for Miriam, who was restored; not, however, till she had been made, by her exclusion, a public example [Nu 12:14, 15].

Matthew Henry's Concise Commentary

12:10-16 The cloud departed, and Miriam became leprous. When God goes, evil comes: expect no good when God departs. Her foul tongue, as Bishop Hall says, was justly punished with a foul face. Aaron, as priest, was judge of the leprosy. He could not pronounce her leprous without trembling, knowing himself to be equally guilty. But if she was thus punished for speaking against Moses, what will become of those who sin against Christ? Aaron, who joined his sister in speaking against Moses, is forced for himself and his sister, to beseech him, and to speak highly of him whom he had so lately blamed. Those who trample upon the saints and servants of God, will one day be glad to make court to them. It is well when rebukes produce confession of sin and repentance. Such offenders, though corrected and disgraced, shall be pardoned. Moses made it appear, that he forgave the injury done him. To this pattern of Moses, and that of our Saviour, who said, Father, forgive them, we must conform. A reason is given for Miriam's being put out of the camp for seven days; because thus she ought to accept the punishment of her sin. When under the tokens of God's displeasure for sin, it becomes us to take shame to ourselves. This hindered the people's progress in their march forward towards Canaan. Many things oppose us, but nothing so hinders us in the way to heaven, as sin.