Don't get me started on why journalists should not be confused with historians. Bill Moyers said it very well in a 29 Oct 2010 lecture: "I am a journalist, not a historian. The difference between a journalist and an historian is that the historian knows the difference."
You do have leverage, and I think you are right to use it to have your cousin clean up his tree. But ultimately it is your cousin's tree, and if it is a mess then that is his problem.