I don't think it is quite the same, however I do understand the sadness Dave feels on learning the news that a sweetheart, someone he loved 53yrs ago passed away while still quite young. It is sad, it's not what you want to happen to someone you thought a lot of 53yrs ago.
However, it has been 53yrs, both you and she moved on, I think it would be inappropriate to contact members of her family after all this time. It's possible your own wife Dave, would understand fully why you feel so upset on learning this sad news, as obviously we want people we are fond of and indeed were once fond of 53yrs ago to have long and happy lives. we would not be caring human beings if we felt otherwise.
I'm 71 the same age as you Dave, I experienced genuine real love just once in my life and we had 49 fantastic years until April when I lost her very suddenly. I really think you have to consider how her family may view it in the event you contacted one of them, they may fully understand, or indeed they may think the opposite, I don't know, she moved on and had a family as did you.
I think the fond memories you had/have for this dear lady, should remain just that.....fond memories, when we hear the sad news that a dear friend or anyone we were once fond off passes away, especially at a young age, inside we feel sad so I don't think what you are feeling is silly, grieving affects each and everyone of us differently.
Your 71 Dave, you are very fortunate you have your dear wife to share your love and you hers, and I truly hope you both go on for many more happy years yet. So yes grieve for the loss, even one from 53yrs ago, and indeed the fond memories you have of that person, but don't let those past memories stop you embracing what you have now, and make every day count Dave.