It sounds as if Moses was the first born son according to Exodus 2:1-2, but then in Exodus 6:20 it lists Aaron and then Moses as brothers. Aaron is listed first so he would be the oldest. Where was Aaron when Moses was put in the basket by his mother? Did Moses and Aaron have the same parents? The Bible also mentions Miriam as Aaron's sister (Exodus 15:20) but why not Moses as well? Could you explain the family relationship of these three?
The three passages to which you refer to about Moses, Aaron, and Miriam are presented below.
Now a man from the house of Levi went and married a daughter of Levi. And the woman conceived and bore a son; and when she saw that he was beautiful, she hid him for three months. But when she could hide him no longer, she got him a wicker basket and covered it over with tar and pitch. Then she put the child into it and set it among the reeds by the bank of the Nile. 4 His sister stood at a distance to find out what would happen to him. Exodus 2:1-4 (NASB)
And Amram married his father’s sister Jochebed, and she bore him Aaron and Moses; and the length of Amram’s life was one hundred and thirty-seven years. Exodus 6:20 (NASB)
And Miriam the prophetess, Aaron’s sister, took the timbrel in her hand, and all the women went out after her with timbrels and with dancing. Exodus 15:20
First, notice that the Bible does not provide any details about Aaron and Miraim in the account of when Moses was placed in the basket that put into the Nile River (Exodus 2:1-4). Scripture never tries to give us all of the details of an account because it is more interested in sending us a message rather than communicating everything. In both of the passages above, we are told that Moses, Aaron, and Miriam are siblings and they have one father. The father’s name was Amram. He was a Levite.
There are two other passages in the Old Testament that clearly tell us these three are brothers and a sister. The first passage is found in Numbers 26 and the second one is found in 1 Chronicles.
And the name of Amram’s wife was Jochebed, the daughter of Levi, who was born to Levi in Egypt; and she bore to Amram: Aaron and Moses and their sister Miriam. Numbers 26:59 (NASB)
And the children of Amram were Aaron, Moses, and Miriam . . . 1 Chronicles 6:3 (NASB)
The first passage says that their father was named Amram and their mother was Jochebed. From Exodus 2:1 we discovered that Amram was a Levite and from Numbers 26 we discover that Jochebed was a Levite too! Together Amram and Jochebed had two sons named Aaron and Moses and one daughter named Miriam. The second passage confirms the statement of the first one.
Moses and Aaron
From both of those passages, we discover that Aaron was the older brother, since he was listed first in each geneaology. It was customary in the Old Testament to list the first born son first. This is great news. God is more concerned about the heart of a person than about their lineage. How did Moses feel when God told him that He was going to select his older brother to speak for him? How did Aaron feel when Moses became the leader of Israel?
The answer to the first question may be found in this verse,
Now the man Moses was very humble, more than any man who was on the face of the earth. Numbers 12:3 (NASB)
Moses was a very humble man. I believe that Moses did not react negatively to his brother’s speaking for God before Moses. What a wonderful example to us. God calls us to serve Him and not worry about the other person. Praise the Lord.