Thanks to all of you who weighed in with opinions and shared your own family situations in support of telling this guy that he has a sister. I think he should know, but Mr. kenju is still unsure. He has read all your comments, and he will discuss it with the only other family member who knows about it, and then he will make his decision whether to tell or not.
The people involved are getting on in years; the woman who had the out-of-wedlock baby girl is surely deceased, and we don't know the name or whereabouts of the half-sister - or indeed if she is alive. If she is not, as someone pointed out, it might be best not to tell.
Each of you brought up some points we had not considered, as I knew you would. I am blown away by the thoughtful kindness you show every day, but especially at times like this. Again, thanks for your willingness to help.
Now please scroll down for a post that might be just a little bit more jolly!