While on holiday I took the opportunity to find some family graves in Dundee. I have discovered that my mother had a brother who I knew nothing about. In fact I was always told she was an only child. He apparently died in 1944 in Aden at the age of 22. Why on earth is that a family secret
I think Aden was a British base during WW2, so maybe there were circumstances surrounding his death that they couldn't/wouldn't discuss.
However, it might not be that it was actively a family secret (i.e. conciously hiding it). Maybe they just didn't discuss him publicly ("out of sight, out of mind" type of thing).
When I started doing my tree, I was told my paternal g-father was one of five, but it turns out one of those was a twin (making six) and one of the twins died young. My dad was stunned.
Also, my OH's father knew he had a half-sister (from his father's first marriage), but we found another - a half-brother - who at first we assumed must have died young, as no-one knew about him. Turned out he had been sent to live with a maternal aunt when OH's g-father's first wife died and stayed in that area, living in to his mid-60's with kids of his own!!