It was indeed a TRAGIC situation. And, I too do not want to sound calous, but these situations are why travel insurance is made available... to cover you against the unknown.
While I understand it is much more personal to you (and I do offer my sympathy) I'm uncertain why the cruise line should be held financially responsible for all the bad things that can happen to people and their families which require them to cancel their trips. Again, that is why travel insurance is avaialable (and it's not very expensive).
As Curtis suggested, I might try and contact Princess again through your travel agent, and try and climb the ladder at the cruise line. You may get a more sympathetic ear, with someone willing to override the rules, and judge these things on a case by case basis.