re: Why is Grand Isle in Jefferson Parish?

Kajungee you are right
In 1788, Spanish governor Esteban Miro granted Isle Longue to Jean Voisin prior to the establishment of Lafourche and Terrebonne Parishes, but then known as Jefferson Parish all west of the Mississippi River; this broad coastal area was then known as Barataria. Jean died in 1820 in Pointe à la Hache, leaving all his properties to his son and daughter. Jean Joseph Voisin and Catherine Voisin. By 1824, Catherine had died and Jean Joseph married and moved onto the island for the fishing industry. Problems with land offices in LA recording in DC, and land on the island was sold and Jean Joseph filed a suit in Terrebonne Parish court, in the late 1840s — which litigation continued until the Civil War, when the courthouse was closed down by Union officials.