The families of the oldest brother were treated the same as those of the youngest.

 1 Chronicles 24:1-31

These were the divisions of the descendants of Aaron: The sons of Aaron were Nadab, Abihu, Eleazar and Ithamar. But Nadab and Abihu died before their father did, and they had no sons; so Eleazar and Ithamar served as the priests.

A larger number of leaders were found among Eleazar’s descendants than among Ithamar’s, and they were divided accordingly: sixteen heads of families from Eleazar’s descendants and eight heads of families from Ithamar’s descendants. They divided them impartially by casting lots, for there were officials of the sanctuary and officials of God among the descendants of both Eleazar and Ithamar.

The scribe Shemaiah son of Nethanel, a Levite, recorded their names in the presence of the king and of the officials: Zadok the priest, Ahimelek son of Abiathar and the heads of families of the priests and of the Levites—one family being taken from Eleazar and then one from Ithamar.

As for the rest of the descendants of Levi: from the sons of Amram: Shubael; from the sons of Shubael: Jehdeiah.

This was their appointed order of ministering when they entered the temple of the Lord, according to the regulations prescribed for them by their ancestor Aaron, as the Lord, the God of Israel, had commanded him.

And the sons of Mushi: Mahli, Eder and Jerimoth.

These were the Levites, according to their families. They also cast lots, just as their relatives the descendants of Aaron did, in the presence of King David and of Zadok, Ahimelek, and the heads of families of the priests and of the Levites. The families of the oldest brother were treated the same as those of the youngest.

Why?
All of the services of Levites are same and they have equal rights. All of them cast lots to accept the service roles.

The descendants of Eleazar were much more than other tribes. they were blessed by the Lord.

There are 24 appointed ministering orders in the temple.

How?

Serve roles before the Lors were equal positions; each role was important, the same as the role in the church service now.