Numbers 16:1 - Wesley's Explanatory Notes

The son of Izhar — Amram's brother, Exodus 6:18, therefore Moses and he were cousin germans. Moreover, Izhar was the second son of Kohath, whereas Elizaphan, whom Moses had preferred before him, and made prince or ruler of the Kohathites, Numbers 3:30, was the son of Uzziel, the fourth son of Kohath. This, the Jewish writers say, made him malcontent, which at last broke forth into sedition. Sons of Reuben — These are drawn into confederacy with Korah, partly because they were his next neighbours, both being encamped on the south — side, partly in hopes to recover their rights of primogeniture, in which the priesthood was comprehended, which was given away from their father.