My father is 93 years old. He does not have a relationship with his younger sibling and has not had any contact with her or her family in over 50 years. He is the middle child of 7 children. Due to circumstances out of his control their family had been separated when they were young. He has slight dementia and his health is declining. He's frequently sad and states he wants to die. He always mentions that he feels guilty that he has lived so long when his other siblings have passed away. He has one younger sibling alive. It was decided by a distant relative in the other country and myself that we would hold back this information because of concern for his mental health and well-being. I am now having second thoughts whether or not this was the right decision. Should he be told and for what purpose?
Not to sound callous, but what happened to the other four deceased siblings, when, and how did he cope with hearing about them?
Does he ever talk about the other branch of his family, or about this individual in particular?
I must admit can't see the slightest benefit to him in saying "you know that sister you had nothing to do with for half a century and never talk about? Well, she's died." But then again, I wouldn't lie. If the sibling or her part of the family tree comes up, and the opportunity felt right, I would say something like "I am afraid we had some sad news about x" and then take my cue from his response as to how far to go into the details.
There is a but. What were the circumstances of the family schism? Political upheavals or something? Are these events much on his mind? How much he is thinking about the past might influence what you feel he needs to know now. If you're concerned about your agreement with the other relative, revisit the conversation and explain that you want to be more flexible in your approach.