Returning to the original question ( I agree that Jen67patte should start a new thread):
The question now is not whether they should be burned or not, but the siblings' argument. Your friend has no hard evidence of her parents' wishes. Whilst burning them would be the right thing to do, she does indeed risk a family rift.
In her shoes, I would say "Please can we put them away in a box /tin, with a note saying something like - parents asked that these be burned but some of their children don't want to destroy this piece of family history-". OK it passes on responsibility, but satisfies everyone for now.
It is not her responsibility that some siblings read them, and she should not worry about that. Her parents should have been clear in their instructions and weren't. Nobody's perfect!