The problem here is that its when the kids grow up and meet each other and they are half siblings. Instead of a simple blood test for marriage, you will need a DNA test to see if you are related. Netflix documentary: 2 kids in elementary school, having different families find out they are half siblings.
Fertility clinics in Europe have this same problem. The thing there is the sperm donors are blocked by the clinics, now go online, and have actual intercourse with the women wanting babies.
Again Tall muscular blonde blue eyed men are perferred ..... oops that is WW2 Germany again.... These women/couples are looking to improve their offsprings genetics.