I'm guessing modern genetic tests can predict the safety of having children even between siblings (because most of the issues are with recessive genes).
I honestly don't know. I suppose if the system could at least run tests for the recessive genes maybe that is sufficient. Even if the issues are strictly taboo it could possibly make for very awkward family gatherings as people start to put together who knew who.
High fives all around
It wasn't that long ago that gay marriage was illegal with, as far as I know, the only real argument being the opinion of "That's gross". With the tests you describe, or permanent sterilization, the only real thing in the way of incestuous marriage is "That's gross". I'm guessing we won't see any court decisions that truly dive into the reasoning for a long time, if ever (in our lifetimes). It is an interesting thought experiment.