Endemism on islands is quite a common phenomenon, at least at the level of species or genera. Old continental fragments like Madagascar or New Caledonia may even harbour endemic families which went extinct on the continents. I also know of the odd example of Lactoridaceae that comprise only one species which is endemic to the Juan Fernandez Islands.
Do you know other instances of endemic families on islands? What about Hawai'i, the Canaries or Galapagos?