This is the family of Aaron and Moses at the time when the Lord spoke to Moses on Mount Sinai.
These are the names of Aaron’s sons: Nadab the firstborn, then Abihu, Eleasar and Ithamar. These are the names of Aaron’s sons who were ordained priests and anointed and were ordained to exercise priesthood.

But Nadab and Abihu died before God, when they offered a strange fire to the Lord in the wilderness of Sinai; they had no sons.

Eleasar and Ithamar practised priesthood under their father Aaron.
And the Lord spoke to Moses and said: Behold, I have taken the Levites from the children of Israel instead of all the firstborn from the wombs of Israel, so that the Levites shall belong to me.

On the day when I struck down all the firstborn in the land of Egypt, I hallowed unto me all the firstborn in Israel, both man and beast, for I am the Lord.