Detail #206: Some Mucking about with Lexemes and Grammatical Number

Dyadic kinship terms are a somewhat interesting lexical detail for which we could create some interesting things.

Consider, for instance, nouns such as cousins. We have a few situations:

they are cousins (reflexive)
they are my cousins (non-reflexive - they need not be cousins of each other)
the two of them are cousins (reflexive)
all of them are cousins (reflexive)
all of them are cousins (partially reflexive - cousins of someone else among them) - essentially, "all of these form sets of cousins"

Now, we could go on with this; some nouns may only have one reasonable interpretation for the ~reflexive plural interpretation: 

father and son →_plural father and sons (a single family unit)

However, we might also want to permit 

father and son →_plural fathers and sons (multiple family units of two or more)

and maybe we want to distinguish this without really bringing in anything like "collective" plurals or anything (at least as a regular thing in the language). So, what if we form one of the plurals irregularly (i.e. unexpected conjugations, suppletion, small stem changes, unusual morphemes?) - but with a twist. For some nouns, the regular form marks the singular set with multiple members, the irregular form marks the multiple sets with pairwise or more members; for other nouns, the opposite occurs.