I think she wants to believe it's true and that;s why she can't defend her beliefs
Who wants to be on the defensive in their own home? That can't be much fun....I'm sure she probably feels outnumbered. Why not casually ask her simple questions once in awhile as if you simply want to understand things better rather than change her mind. Simply ask casual questions and accept whatever her answer is without comment.
Example:
You: "I've forgotten the Bible reasoning behind "headship". I remember that for some reason women are supposed to be in subjection to men and yet they can be part of the144,000 and will rule with Jesus and Judge mankind once they get to heaven but they can't even run the microphones at the Kingdom Hall here on earth."
Her: "The Bible says at Ephesians 5 that Women are to be in subjection to their husbands."
You: "Ok...so they're to be in subjection to their husbands but I was never clear on how that plays into having responsibilities at the Kingdom Hall. It's probably not all that important, I was just thinking about it. Anyway...pass the potatoes please."
Often, in the process of answering a question, they can see the flaws in the JW reasoning on their own without having their noses rubbed in it. That way get can get comfortable questioning their understandings and can do so without feeling they need to defend them. That's what I did and it worked immediately.