Today I saw an interview of Michael J. Fox on the Rachael Ray Show. Rachael asked him the key to happiness and harmony in marriage and he said that (someone told him) "if a lady 'says' something then she is going to be right". I realized as he said it that he touched a sensitive nerve.
If she says something emphatically, that she feels strongly about, then her husband should try if at all possible to just let it go. Even if he does not agree, he has to laugh. Even if the whole family knows she's probably wrong, the best thing to do is to just relax and pick another battle. Just publicly agree. (The entire family might even privately agree she's wrong, too, don't worry!).
She needs to save face and appear to have some power, too, and this is her big chance to gain the support and approval that feels to her like love. That is how a woman feels, and with her good graces, how a family achieves harmony.
Besides, she might just be right.