Siblings, relatives (even generational) == already intimately acquainted.
Social conventions are inextricable from the situation, prima facie -- else nose-rubbing would have been an option, as well as ...

For no one to have shaken the same number of hands, a one might postulate the number of kin pairings (including possible ex-es)-- but that would render merely a probability (n! reasoning) and not properly fill a Truth table.

(Likely Mr. Fibonacci would spawn a better diagrammatic exercise, as more information dribbled in?)

This one seems akin to one ~ about a boy in car accident, now in surgery; the surgeon exclaims, That's my Son!! (yet the surgeon is not the father, per intro.)