They must be afraid of same sex conception?
I dunno,
Their kids will come out deformed!
If the married mother and daughter also marry the son/brother, he can impregnate them both.
Once perversion was made legal it was never going to stop with same sex marriage. Perversion is going to run the table.
Yet you apply a standard to the Mother/Son that you don't apply to the Mother/daughter. That seems to me to fly in the face of current legal theory. Especially when you factor in the fact that sex is not a requirement to marry?
Do I support legal family marriage? Absolutely not, but given the current legal standards, I don't see a reason legally that it won't at some time be legal.
If it has not changed recently, the marriage law in the state of Iowa actually lists those pairings that are prohibited from marriage. Last time I checked, same sex family members were not on that list (but that was roughly 6 months ago that I last checked).