Isn't avuncular marriage considered incest in Judaism? Doesn't that mean that Moses was a product of incest since his mother was married to her nephew?
Avunculate marriage is marriage between an uncle and a niece or between an aunt and a nephew.
The latter is Biblically forbidden (Leviticus 18:12-3). The former, however, is not, and the Talmud actually encourages it (Yevamot 62b-63a).
Interestingly, R. Saul Lieberman z"l contends that the Talmudic encouragement for marrying one's niece, was specifically meant to counter a Sadduceean position that banned marriage to nieces, in spite of the lack of explicit Biblical prohibition. (Tosefta Ki-fshutah (New York: JTS, 1973) pg. 915).
Regarding Moses, he was indeed the product of a union that was subsequently forbidden after the Law was given. (Exod. 6:20). Regarding a possible violation of Noahide incest laws, some state that she was a paternal aunt, rather than a maternal aunt. (Yalkut Shim'oni B'reshit: 25, cf. Rashi Exod. 6:20). Others are of the opinion that Noahide law does not include a prohibition to marry an aunt (cf. Sanhedrin 58a, Hilkhot Issurei Biah 14:10).
Interestingly, Hizkuni suggest in his commentary to (Exod. 6:20) that Moses' lineage wasn't a bug; it was a feature! It is good for a leader to have some stain in his past to prevent him from excessively glorying over his subjects.
For a lengthy discussion of all things avuncular and Jewish, see here.