The Biblical text does not indicate here whether this was a marriage which had taken place some time previously or whether this was a recent event. The Hebrew commentary, Soncino, offers one possible explanation that the Cushite woman was “a woman of Ethiopian origin.” It continues:”Legend tells that Moses married the queen of Ethiopia…”
The Hebrew writer Josephus gives the following narrative in his work, “Antiquities of the Jews”, p. 58, addressing one of Moses’ campaigns as an officer in Pharaoh’s army, prior to his flight from Egypt (compare Acts 7:22-29; Hebrews 11:24-27):
“Tharbis was the daughter of the king of the Ethiopians: she happened to see Moses as he led the army near the walls, and fought with great courage; and admiring the subtlety of his undertakings, and believing him to be the author of the Egyptians’ success, when they had before despaired of recovering their liberty, and to be the occasion of the great danger of the Ethiopians were in, whence they had before boasted of their great achievements, she fell deeply in love with him; and upon the prevalency of that passion, sent to him the most beautiful of all her servants to discourse with him about their marriage. He thereupon accepted the offer, on condition she would produce the delivering up of the city; and gave her the assurance of an oath to take her to his wife; and that when he had once taken possession of the city, he would not break his oath to her. No sooner was the agreement made, but it took effect immediately; and when Moses had cut off the Ethiopians, he gave thanks to God, and consummated his marriage, and led the Egyptians back to their own land.”
If this narrative is based on truth, and if Miriam and Aaron brought up Mose’s alleged marriage with Tharbis in Numbers 12:1, then it would be very clear why the “anger of the LORD was kindled against them” (verse 9) — after all, Moses would have married the princess before his flight to Midian, that is, long before his conversion.
There is, however, another possibility as to who the “Ethiopian woman” might have been. Soncino continues to explain: “[A commentary] identifies the Cushite woman with Zipporah [whom Moses married while in Midian, after he had escaped from Egypt, compare Exodus 2:21] who was a native of Midian. The Midianites, who were tent-dwellers and dark-skinned, were also known as ‘Kushim.'”
The Midianites were descendants of Abraham and Keturah (Genesis 25:1). Abraham took Keturah as his wife (same reference) or concubine (1 Chronicles 1:32) after the death of Sarah.
The Broadman Bible Commentary has this to say about the “Cushite woman”:
“The identify of the Cushite woman has been widely debated. The only known name of a wife of Moses was Zipporah (Ex. 2:16-22; 4:25; 18:2). However, there are times here and elsewhere at which Moses’ wife is referred to without specific name. It may be that the writer is referring to Zipporah here… For a long time, Zipporah had been left with her father (along with Moses’ two sons) but Jethro brought them to Moses. While Zipporah and the sons were absent, Miriam and Aaron had no challenger for second place; but when they were present there was a constant reminder of the several suggestions which had come through the Midianites upsetting the status quo arrangement. Zipporah was a Midianite (Ex. 2:16) or Kenite (Judg. 1:16; 4:11). In Habakkuk 3:7 the term ‘Cusham’ and ‘Midian’ are used in synonymous parallels. So she may have been referred to accurately as a Cushite woman. The story of Zipporah records the fact that Moses sent her away (Ex. 18:2). This is the same term as is used elsewhere for divorce. In the same chapter (Ex. 18:1,6) we are told that Jethro, still called the father-in-law of Moses, brought Zipporah to Moses ‘in the wilderness where he was encamped at the mountain of God’ (Ex. 18:5) along with her two sons. This account indicates (Ex. 18:27) that later ‘Moses let his father-in-law depart’ to his own country. No mention is made of Zipporah and the two sons, Gershom and Eliezer. Thus it may be that the Cushite woman was Zipporah. The reference in this case would explain a repeated reference to Cushite to emphasize with some disdain that Miriam and Aaron considered her a ‘foreigner.'”
This explanation would also shed light on the fact that God’s anger was kindled against them, and mainly Miriam, the apparent “spokesperson” in the incident. Verse 2 reports that they murmured against Moses, saying, “Has the LORD indeed spoken only through Moses [“and” — following the comments of the Broadman Bible Commentary — “Zipporah, that Cushite woman, who is influencing Moses”]? Has He not spoken through US ALSO?”
God, however, was not pleased with this criticism. He told Miriam and Aaron: “I speak with him [Moses] face to face, Even plainly, and not in dark sayings; And he sees the form of the LORD. Why then were you not afraid To speak against My servant Moses?” (verse 8).
Although we don’t hear anything further about Zipporah (if she was indeed the Cushite woman in Numbers 12:1), the Bible may contain a later possible reference to Moses’ first-born son, Gershom, in the book of Judges. We read that the Danites engaged in idolatry, setting up for themselves a carved image, and “Jonathan the son of Gershom, the son of Manasseh [in the margin, an alternate rendering is given as “Moses”], and his sons were priests to the tribe of Dan” (Judges 18:30). The Hebrew Tanakh points out that “an earlier reading [i.e.,] ‘Moses'” instead of Manasseh, is “indicated.” The Ryrie Study Bible comments that the better rendering is “son of Moses. The Danite priests traced their lineage to Moses.” Most German translations, including Luther, Zuercher, Menge and Elberfelder, consistently render this phrase as, “son of Moses.” If the reference in Judges 18:30 to Gershom is indeed a reference to the son of Moses, then Moses’ and Zipporah’s grandson Jonathan had begun to be deeply involved in idolatry.
We find an additional reference to Gershom, the son of Moses, in 1 Chronicles 23:14-16; 26:24. These passages mention Shebuel, a son of Gershom, the son of Moses, who had become overseer of the treasuries. This grandson of Moses seemed to have stayed loyal to God’s way of life. We also learn, in 1 Chronicles 23:15, 17, that Moses’ second son, Eliezer, had a son, named Rehabiah, and that “the sons of Rehabiah were very many.”
In any event, whether the “Cushite woman” in Numbers 12:1 was a princess of Ethiopia or Zipporah, Miriam and Aaron should have never used Moses’ marriage as a justification to develop feelings of self importance and envy, resulting in their murmuring against Moses. They later acknowledged and repented of their sin, so that God could continue to use them in His great plan (verse 11).