Just treat "nother" as a whole nother word. It's a jocular formation from the last part of adjectival "another" that has become common. The problems with defining it as a variant of "another" come at the morpho-syntactic level: how could you explain that only this variant of "another" invariably is preceded by "whole" and/or can be the modifier of the head noun of a phrase beginning with the indefinite article? A necessary condition for defining lexeme A as a variant of lexeme B instead of an independent word is that they behave identically in syntax. For example, the 2-syllable and 3-syllable prons of "family" can be treated as variants of 1 word because no syntactic language model could reliably distinguish them. Contrariwise, if you have different symbols for "nother" and "another" and train up a word-level language model with them, you'll find that their probability distributions over ngrams are quite different.

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