I've also seen someone suggest that "ba'buir" incorporates a kind of reduced reduplication (I don't know the technical terms). In other words, "ba'buir" comes from "buir'buir," which became "bu'buir," which became "ba'buir," because the author, er, some important person in the history of the Mandalorian language liked that sound better.
As for "cousin" or "nephew/niece": I've seen plenty, because many people believe that Mandalorians would copy the European nuclear family structure. I don't keep track of them, because I prefer the fan proposal that Mandalorians use an alternate naming system for kinship relationships. For example: https://64.media.tumblr.com/925ad54cb43 ... 81b6cb.jpg
A discussion of that tree is in this Tumblr post: https://www.tumblr.com/ranahan/77219062 ... alled-gimp