Moses had married Zipporah—one of the seven daughters of Jethro, the priest of Midian—when he fled from Egypt after slaying an Egyptian taskmaster (see Exod. 2:21). Forty years passed, and God called Moses back to Egypt to deliver Israel. On his journey he took Zipporah and his son Gershom, who was uncircumcised. God’s anger led to Zipporah suddenly circumcising her son and accusing Moses of being a “blood husband” (meaning, “the marriage bond between us is now sealed by blood”). But Zipporah sees the shedding of blood in circumcision as unnecessary because she was unfamiliar with the Jewish law of circumcision. After circumcising her son at an inn, Zipporah became angry and she and her son went back to her father. God “let him go” (Exod. 4:26), meaning that she returned to her father and sent Moses alone to meet Aaron.
Zipporah was a Midianite (see Exod. 2:16), a descendant of Abraham through his wife, Keturah (see Gen. 25:1-2). After Israel departed from Egypt, Jethro brought Moses’ two sons and his wife, Zipporah, into the wilderness to be reunited with Moses (see Exod. 18:2-6). After the meeting, we read that his father-in-law eventually “went his way into his own land” (Exod. 18:27).
Here in Numbers, Miriam and Aaron criticize the Ethiopian woman that Moses had married. Some rabbis suggest that Moses divorced Zipporah, as she is not mentioned again, and that Aaron and Miriam are upset because Moses remarried this Ethiopian woman, whom he had married when he lived in Egypt.
However, the answer may be found in the writings of Josephus. According to Josephus, the first forty years Moses lived in Egypt, Moses defeated the King of Ethiopia in a battle and married the king’s daughter, named Tharbis (Josephus, Antiquities of the Jews, Book II, chapter 10). When Moses suddenly departed from Egypt at age forty, he left Tharbis behind. Believing he would never return to Egypt, Moses married Zipporah, a daughter of Jethro. Forty years later, Moses returned and brought the people out. Some suggest Moses may have brought his former wife, Tharbis, out of Egypt with him when he departed. Others believe that when Moses left Egypt, Tharbis returned to her land in Ethiopia.
Some rabbis suggest that this woman simply refers to Zipporah, and Aaron and Miriam were concerned that after Moses’ death, he would place his “non-Hebrew” sons born in the land of Midian over the nation. If this woman mentioned here was Moses’ early wife from Ethiopia, then this is perhaps why Miriam criticized him. It is conjuncture to attempt to prove which theory is correct.