5 Now the law requires the descendants of Levi who become priests to collect a tenth from the people—that is, from their fellow Israelites—even though they also are descended from Abraham.

Read Hebrews 7:5 Using Other Translations

And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham:
And those descendants of Levi who receive the priestly office have a commandment in the law to take tithes from the people, that is, from their brothers, though these also are descended from Abraham.
Now the law of Moses required that the priests, who are descendants of Levi, must collect a tithe from the rest of the people of Israel, who are also descendants of Abraham.