I wonder what the risk of inbreeding is between a grandparent and a non-incest-produced grandchild.
For example, first cousins can marry and have children with little risk of genetic disorder. But then the resulting children can’t have children with THEIR first cousins, as the genetic similarity is too close.
I imagine that if your family history has had recent incest (people, incest tends to be a lot more common than you would think in history and chances are it’s there in your family tree) the risk from genetic damage from inbreeding would be a lot more severe. But I’m the case of no habit of inbreeding and your grandparents are magically made young, I can’t help but wonder about the math and genetics.