Nothing you can do. There's no way to get those messages back. Remind your sis that dear friends haven't gone away. A message is much like an object we think we need to keep because it reminds of of someone or is from someone. When those things are lost, or gone for any reason, what's important to remember is the people they remind us of are in our lives, memories and in our hearts. That NEVER goes away. No one can take those things, or delete them... ever! In reality, messages can't be stored on a phone forever anyway. There is a limit to how much can be stored. It might have been a blessing in disguise that they were deleted. Saving things can get out of control.