You: I wouldn't forbid it. People can do whatever they want, as long as it does not affect me. 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.
Thanks for your honesty. Now I'll be honest. You are a sick deviant, and I sincerely hope you don't have any interactions with children in your life.
If, for example, father-daughter or father-son "marriages" were legalized and recognized, you would have numerous sick perverts raise their children for the purpose of seduction. As soon as the son or daughter hits 18, presto, "marriage." It would be a pervert's dream come true, and you support it.
Seek help.
I used to be a teacher. Really. I left because the administration and the other teachers were too liberal, and the school system was broken in general.