Odd assumption that everyone has grandparents alive.
And I suspect disaster to have correlated chances of killing parents or grandparents or children. i.e. if you've become a parent there's a good chance your parents did too.
Our own lived experiences are likely outliers, since developed countries have different outcomes than the past.
Crap and nonsense! Rewrite:
Once someone's survived to become a parent, there's a good chance the grandparents survived as well (assuming the norm is to breed ASAP).