My grandfather constantly had arguments at work and lost his job. He had a second family later in life and withdrew to be become a Stay at home dad in his early 50s. This was very unusual in the late 70s. His second wife was the bread winner.
He became increasingly reclusive and withdraw. He hated changes to his routine. To the point that he would actively refuse to arrange to see my Dad. (When we were going up, I think I saw him on about 4 occasions). This got worse as he got old.
He died a couple of years ago in his 90s. My parents have found out more about how bad it was since then.
A good example is how he recorded everything he bought. Date, time, location, price. For years. He made lists and list and lists and kept them. Obsessively so.
His second son, is only a couple of years older than me in his late 40s. He is obsessive about football and cricket scores. He doesn't want to actually go and see matches. He just likes the numbers. He struggles to look after himself (only briefly lived independently) without prompting from his elderly mother. Financially he can't manage his own money. My parents are extremely worried about how he will cope when his mother is no longer around.
Neither my grandfather nor my uncle were/are diagnosed. We strongly suspect long family history of autism in the family and potentially ADHD. But we can't prove it - but theres a catalog of behaviours in their that tick a lot of boxes which we can actively prove with actual evidence (my dad has kept some of his father's lists - but cleared out a large number).
Interestingly my grandfather was a twin - his mother was 41 when she had him and my grandfather and his second wife were both older when they had my uncle too. So they do tick those boxes as well.
I think there will be a LOT of people who will have similar family histories. But no diagnosis.