Moses had asked God to spare the people, but now he called on those who followed the Lord to kill those who had sinned. The Levites (32:26) were willing to confront the sin that Aaron had let loose. Although Moses commanded them to kill everyone (32:27), the number 3,000 (32:28) makes it clear that the words each of you had a restricted meaning.
The reference to his son and his brother suggests that as Aaron had led in the idolatry, many of the Levites had led in the worship of the idol, and they were the ones that the rest of the Levites killed. Aaron may have escaped because God had already designated him as high priest (28:1).