I think that you've gotten some excellent suggestions for the letter, and I want to add one more little tidbit.
When I went to Hawaii many years ago, I was seated on the plane next to a couple that were celebrating their 50th anniversary in Hawaii, courtesy of their children. They were thrilled to death to be going, because they thought that they would never be able to see the islands. And they were so grateful that their kids gave them that 'once in a lifetime' chance.
Good luck with your siblings!!