Oh I see what you’re saying. I did some reading earlier that said that in a lot of places 20% - 40% of all marriages are to first cousins.

If you have one person with recessive genes and one person with dominant genes, then the baby will have the dominant gene. So if the grandparents were cousins both with recessive genes it wouldn’t matter, as far as I know.

But it’s not a 15% risk. Unrelated couples have a 3% chance of having a child with a birth defect while cousins have a 5% chance of having a child with a birth defect.