My grandparents on my mom's side are both dead allready but back when I was a small kid my grandma and granpa were very cold to eachother, even slept in seperate bedrooms and never spend much time together. I of cause asked as a kid why they were not like other grandparents and they never liked to talk to me about that.
I just figured out years later what the issue was. My grandma was a doctor with her own office, my grandpa a judge. Both lived in a street full of other snob like people and wanted to keep a "good family image" by not divorcing from eachother for the shit my grandpa did. He betrayed my grandma and she acted up as if she wouldn't notice. Each time my grandpa said he takes his caravan to a 2 week trip to Spain to go "surfing" at the beach, he actually just drove to the next big city, parked in a camping area and than went to some other older women he was secretly meeting.
Nobody outside the family ever knew about that and as my grandma passed away 5 years ago I was quit forward and wrote a not so nice letter about my family with a card that you send to people when someone passed away to tell them that you actually feel sorry for their los of someone. I never was even in good terms with my grandparents to begin with so telling them after not seeing in ages that I knew about this and that this was also one reason why they bullied my mom (who knew about it before my grandma knew about that. I mean my mom was bullied from my grandpa to not tell my grandma and that made my mom psychic ill over the years)
well thats the darkest family secret I have