But only two at a time? Why? On what basis?
Also, would you forbid two brothers marrying? A mother and a daughter over 18? Why or why not would you forbid a father and a daughter over 18 from marrying? On what basis would you forbid it?
Once the definition of marriage is destroyed, there is no basis for forbidding any of the above. They will have to be allowed, on the basis of "fairness."
I suspect we have been trolled.
Also, I really don't think that there is going to be a huge push for father-daughter or brother-brother marriages. But if there are, then good luck to them.