When it comes to sea sickness in average sea conditions, lie to her... lie like you have never lied before. It is a risk, but it has paid off for me.
Really the mental part of sea sickness is the worst for those that get sea sick. They are worried about it, thinking about it, then they wonder if they are starting to feel it, then not before long, it hits them. I have found that the more you can convince them that they are NOT going to get sick, the less likely they can convince themselves that they are going to get sick.
I keep some of the bands on my boat... when someone seems like they are turning green I bust them out and give them the hard sell. "Oh these bands are literally the only thing I will ever use on my boat, they work 100% of the time, they are amazing, worth their weight in gold. I have been out chasing tuna in 18' seas and 40 knt winds and felt amazing the second I put one on... put it on your will feel better immediately. See you look much better, the color is coming back to your face, would you like to drink this coke?".
It has worked well for me in the past... when it is a buddy, that is usually their last invite though. You don't have that option with your wife. Tell her whatever you need to tell her to make her believe it was something else, and not her that was making her sick... and that you have the cure!