Among
J.R.R. Tolkien's Hobbits, "twelve-mile cousin" was an expression for a person who fulfilled his or her responsibilities only as far as the letter of law or custom mandated.
A Hobbit celebrating his or her birthday both gave and received presents. In Shire custom, only family (second cousins or nearer) who lived within 12 miles were customarily expected to give gifts on their kin's birthdays. A twelve-mile cousin would refuse to give gifts to anyone who did not live under 12 miles away, according to his or her own reckoning.
This expression is given in a footnote to letter #214 in The Letters of J.R.R Tolkien, which discusses Hobbit birthday ceremonies.