Basic Premise of the Movie is Wrong

The woman was persuaded by her MIL to have sex with her BIL in order to become pregnant. MIL says it's ok because it's all "the same blood". Woman also uses this argument to convince her husband that her adultery with his brother was ok.

What is wrong is that the woman did not discuss this with her husband beforehand to see if he would agree to this. Trying to convince him after the fact is ludicrous.