Yeah but will everyone be required to take a DNA test before marriage? What if Joe and Susie's genetic offspring(the ones they donated-not siblings known to each other) get jiggy on Spring Break, and the girl finds herself pregnant? Ooops, that's what abortion is for. /sarcasm
What happens if the offspring decide to do the right thing and get married, and raise the child? None of this is beyond the realm of possibility, imo. The smart, rich people float in their own environment, as do poor people, illegals, and criminals.
I won't crack wise on this one -- As a matter of a very prudent and commonsense precaution, I would advise everyone to have a complete family medical history prior to conception. Geneticists can easily work out the odds of some horrible disease. Unfortunately, this is rarely done -- since it isn't covered by health insurance.
That said, the risk of some terrible disease befalling a kid conceived the old fashion way of bumping bellies is far greater than the risk of half-siblings meeting and falling in love.