Good for her, and good for you.
My wife of 25 years did not sit for my parents' heroic family portrait because she can't stand them. When I first introduced her to them, the first thing my mother did was ask me all about all the other girls who had been calling me.
They get along well enough, but this was just too much for my wife. Fair enough. I sat, and didn't even try to explain her absence, I just told them to ask her, that she had the right to make up her own mind, and that I did not consider it an affront to myself, or an undeserved affront to my mother in particular.
And now, 5 years later, everything's the same as it ever was.
Stand by your wife, and her decision. Feelings, and treatment, matter. Parents are not immune from being a-holes. There need to be consequences for actions. The only reason my wife was invited in the first place was to have more people in the photo, thereby rendering it more heroic. What a joke.
And, it cost me thousands in travel and time off, to make a visit home, as programmed by my moronic sister.
Good for your wife. She should make her decision, and you should make yours, and you should understand each other. There's enough bullshxt in this world, make sure there is none between the two of you.