I had a somewhat strange thought, but wanted to preserve it for posterity. Assume that choten means male relative related by marriage -- thus, male in-law. We need to say something akin to this for choten to mean brother-in-law.
If so, how is Moshe related to Yisro? Simple. Yisro married Moshe's ex-wife and adopted his two children. This is a possible straightforward reading of a pasuk that sets out to tell us their relationship. From the beginning of parshat Yitro:
choten Moshe, the next pasuk defines this relationship and declares:
וַיִּקַּח, יִתְרוֹ חֹתֵן מֹשֶׁה, אֶת-צִפֹּרָה, אֵשֶׁת מֹשֶׁה--אַחַר, שִׁלּוּחֶיהָ
What is meant by שִׁלּוּחֶיהָ and what is meant by וַיִּקַּח? We find both terms used, to define marriage and divorce, in parshat Ki Teitzei, Devarim 24:1:
Perhaps, then, this pasuk in Yisro is stating that after Moshe divorced Tzipporah, Yisro took her as a wife.
This would also clarify a co-indexation issue, in which the pasuk appears ambiguous and, I've always though, multivalent.
his sons and his wife." One straightforwarding reading would be Yitro's sons and Yitro's wife, meaning that he went there to stay. An only-slightly less straightforward reading, which makes a lot more sense in context, is that it refers to Moshe's sons and Moshe's wife. After all, in the next pasuk , Yitro announces them:
|ו וַיֹּאמֶר, אֶל-מֹשֶׁה, אֲנִי חֹתֶנְךָ יִתְרוֹ, בָּא אֵלֶיךָ; וְאִשְׁתְּךָ--וּשְׁנֵי בָנֶיהָ, עִמָּהּ.||6 and he said unto Moses: 'I thy father-in-law Jethro am coming unto thee, and thy wife, and her two sons with her.'|
However, now we can claim that this was multivalent, and it implies both Yitro's wife and sons and Moshe's wife and sons, who are the same people. The sons would be adopted sons. And Yitro is perhaps renouncing claim to them all, saying that Moshe can take them back if he wishes. Or perhaps he is just stressing their historical connection to Moshe.
We do not know that Tzipporah stayed when Yitro left. Perhaps she went with Yitro. But if Moshe took her back, then it would be a violation of the laws in Devarim just cited:
Another major problem - well before this, Yitro (and Yeter) is referred to as choten Moshe! We might be able to answer that this is a retrojection. Since he would eventually be choten Moshe, it mentions it even earlier, in anticipation.