I Believe Moses' Wife Zipporah Was That Ethiopian in Numbers 12

There have been a number of arguments on who the Ethiopian woman in Numbers 12 was. Some argue that Moses may have had multiple wives. Some suggest that maybe Zipporah died (yet Scripture is silent on how she died) and that he remarried for whatever reasons even if she was over 80 years old. Zipporah was the only wife that Moses was recorded in the genealogy. 

There's one thing we need to remember. Moses had married in his 40s and only returned to Egypt when he was 80 years old. What was interesting was that Exodus 18:10-11 shows that Jethro was actually a pagan priest. He only became a believer in Exodus 18. Zipporah may have converted to the faith some time but her father was still an unbeliever. Also, even if Moses was married to Zipporah but you don't expect Miriam and Aaron to know their sister-in-law until they met her later in Exodus 18.

Why do I believe that the Ethiopian refers to Zipporah and not to another wife? It's possible that both Aaron and Miriam held their silence for some time. Sometimes, some people aren't quick to raise their perceived flaws until later. 

An interesting insight from commentaries could help. Here's what the Commentary Critical and Explanatory on the Whole Bible says this of the Ethiopian woman:

Numbers 12:1 - an Ethiopian woman - Hebrew "a Cushite woman" - Arabia was usually called in Scripture the land of Cush, its inhabitants being the descendants of that son of Ham (see on Exodus 2:5) and being accounted generally a vile and contemptible race (see on Amos 9:7). The occasion of this seditious outbreak on the part of Miriam and Aaron against Moses was the great change made in government by the adoption of seventy rulers [Numbers 11:16]. Their irritating disparagement of his wife (who in all probability was Zipporah [Exodus 2:21] and not a second wife he had recently married) arose from jealousy of the relatives, through whose influence the innovation had been first made (Exodus 18:13-26) while they were overlooked and neglected. Miriam is mentioned before Aaron as being the chief instigator and leader of the sedition. 

John Gill also wrote of why he believes Zipporah was that Ethiopian woman:

F8 says he married, when he was sent upon an expedition against the Ethiopians, while he was in Pharaoh's court; nor the widow of an Ethiopian king whom he married after his death, when he fled from Pharaoh into Ethiopia, and was made a king there, as say some Jewish writers F9: for there is no reason to believe he was married before he went to Midian; nor was this some Ethiopian woman he had married since, and but lately, Zipporah being dead or divorced, as some have fancied; but it was Zipporah herself, as Aben Ezra, Ben Melech, and so the Jerusalem Targum, which represents her not as truly an Ethiopian, but so called, because she was like to one; indeed she was really one; not a native of Ethiopia, the country of the Abyssines, but she was a Cushite, a native of Arabia Chusea, in which country Midian was, from whence she came; hence the tents, of Cushan, and the curtains of Midian, are spoken of together, ( Habakkuk 3:7 ) . Now it was not on account of Moses's marriage with her that they spoke against him, for that was an affair transacted in Midian some years ago, which at first sight may seem to be the case; nor because he now had divorced her, as Jarchi, which perhaps would have given them no uneasiness; and for the same reason, not because he abstained from conversation with her, that he might give up himself to the service of God in his house, and perform it in a more holy and faithful manner, which is the common sentiment of the Jewish writers: but rather, as it is thought by others, because of a suspicion they had entertained, that she had interested herself in the affair of the choice of the seventy elders, and had prevailed upon Moses to put in such and such persons into the list she had a mind to serve; at least this seems to be the case, for the displeasure was against Moses himself; they were angry with him, because he transacted that affair without them, and chose whom he pleased, without consulting them; and therefore, though they cared not to ascribe it entirely to him, and his neglect of them, they imputed it to his wife, as if she had over persuaded him, or her brother through her means, to take such a step as he did.

I would agree that there's the possibility of jealousy. Maybe, they never intended to at first speak against it until the incident when Jethro (who had become a proselyte to the faith) provided Moses more useful insights in governance. It's highly possible that they couldn't take it anymore and they decided to rebel. Not all rebellions come out immediately like Korah's rebellion in Numbers 16 wasn't launched immediately either.