When Charles McGrath wrote about the annual Island Cup game between the Martha’s Vineyard and Nantucket football teams for the New Yorker in 1984, he likened it to a fierce sibling rivalry. What mainland team could hope to drum up a rivalry as poignant with either of the Island squads? For all that the Vineyarders can’t stand about the Whalers, they also know that the only football team in the entire country that could possibly understand what it means to be an Islander is that of their brother-in-isolation, Nantucket.