The point is that familial secrets are very common. Yet also common knowledge among family members. It is always the "secret shame" or "that thing too painful to mention" or any number of reasons such things get suppressed.
A couple of cool examples in my family:
1) I'm already married and in my 20s when my wife points out to me that my sister (my parents' first child) had been born months too early to have been conceived on their wedding night. So much of the weirdness in my family suddenly fell into sensible place in that moment. I had just never cared to do the math between their wedding date and her birth date. This, of course, would have been common knowledge to every grown-up family friend and relative I had met in my youth, but I hadn't the foggiest notion of it.
2) My dad ~70 at the time relayed the story one night of how his mother would routinely set an extra place at her dinner table. The place setting wasn't described as something like "for all the hungry people out there tonight" or something like that. He said that his mother always said that the extra place setting was for "Brian."
Me: "Wait, did you have a brother who died in childbirth or something?!"
Dad: "No."
Me: (To myself: Holy crap, my dad has no idea that he had a brother who died in childbirth!)
^No confirmation on a long-lost uncle of mine, but it sure sounds like it to me. Healthy childbirth was by no means a given in the 1930s. If true, this is something that would've been known to all of my dad's older relatives and his parents' friends, perhaps nurses, parish priests, etc., but completely unknown to him his entire life.