still a bit confused even after reading slowly :)

but I do know that you have to also take into account the randomness of the DNA you inherit.

Brads children inherit 50% of his DNA and 50% of their mother (his wifes) DNA -
Then their children (Brads Grandchildren) inherit only 50% of that DNA plus 50% of their other parent.
Theoretically the 50% they inherit (from the parent who was a child of Brad) could be all related to Brads Wife.