I have some personal experience with this. A few months before my grandmother died, she told my father that her husband (his father, as far as he knew) was not his biological father. Apparently she had been dating two different men and got pregnant. One of them offered to marry her and she accepted. It’s not clear if he knew at the time of the marriage that the child wasn’t his, but he found out eventually and had apparently known for a long time when my dad found out. His biological father was already dead (though was someone my dad had been acquainted with - small town and all that) and the father who raised him (I’ll refer to him as my grandfather from now on) was still alive. My dad had been suspicious for many years, but had always put aside any doubts.
The results were mixed. My dad made efforts to connect with his 3 new-found half-siblings, who he had actually known growing up and gone to school with (again, small town). Two of them still live in the same town and my dad is now very close with his “new” brothers. They were all very welcoming and, as it turned out, their own father had told them about the situation nearly 20 years earlier and they had been biting their tongues every time they ran into my dad at restaurants, the barber shop, high school football games, etc. Their mother was still alive though, and I think she felt uncomfortable when they started to invite my parents over for family events.
Although my dad hit it off with his new siblings, this caused problems with his old siblings, who now felt they were somehow being put aside, and to some extent they had a point. And it caused very big problems between my dad and my grandfather. When my dad confronted him about it, my grandfather was in a bad emotional state (his wife was dying) and the resurfacing of the issue and all the surrounding emotions caused him to say some pretty awful things about my grandmother. He apologized at a later point, but the damage was done, and my father and grandfather, despite not being biologically related, share the trait of stubbornness. They hardly spoke to each other for the next 5 years and didn’t reconcile until just a few weeks before my grandfather himself died.
I was only a marginal player in all this, but my wife and I were affected as well. Because of my dad’s feuding with his old siblings, we stopped getting invited to family holiday parties and I lost contact with those aunts, uncles, and cousins for several years. Many of them didn’t come to our wedding because of their problems with my dad. I still made a point to see my grandfather whenever I was back in town, though I knew my dad didn’t approve. Fortunately, my wife and I happened to be back in town just six days before my grandfather died and we had a wonderful visit with him for several hours. We commented on the drive home that we both thought it would be the last time we saw him, and sure enough we got the call a week later.
Anyway, I guess the point of this long story is to be prepared for A LOT of fallout if this secret is ever revealed. Some of it might actually be good, but much of it probably won’t.