Or HaChaim on Exodus 32:27/אור החיים על שמות ל״ב:כ״ז

1 איש את אחיו…ואיש את קרובו. "every man his brother …or his relative." We need to explore why the Torah had to add the words "and each man his relative," seeing the Torah had already authorised the Levites to kill even brothers who are the closest relatives. We have to remember that there are different categories of "brothers" whose outlook on life and whose lifestyles may be drastically different from one another although they are the sons of the same parents. On the other hand, there are people who are not at all related to each other by blood-ties and yet share the same outlook on life and the same lifestyle. All of this is due to the origin of the respective souls of these people rather than to the origin of their bodies. There are souls whose roots are very close to each other and who become separated from one another by being assigned to bodies which are far apart from one another. The reverse is true also. This is why Moses' instructions to the Levites had to include both such possibilities.