Hi there, I've been working in a small newly opened company for two years. I work with a team of 4 of which and I (actually all of us) have a problem with one who happens to be the boss's wife, she is literally an adult kid! Every silly conflict that happens in the team, she directly tells the boss and the project manager with a lot of exaggeration. They don't pay much attention to her (as they say), but every once in a while we get emails asking us to grow up, act professionally, and cooperate with her! The management truly think that we're not mature and I don't know what else because of what she says behind our back.
How to deal with her!

You know the saying, keep your friends close and your enemies closer? I'd try to be friends with her, at least while at work. Get on her good side. As a friend, you might be able to reason with her better. And if that doesn't work, I'd look for another job. The bosses wife will most likely always have his ear and his support.

Of all the problems you can have at any given job, this doesn't sound even in the top 50%.
I'd let it roll off my back.
The emails telling you how to act may not be serious at all; just designed to appease the owner, who may be just trying to give put his wife on the payroll.
If it gets worse, all you others could draft a few sentences (keep it short!) saying that you work well as a team by occasionally arguing and having healthy conflict as a normal part of getting the job done.

Rise above it.

Be professional and polite do the job your getting paid to do and you can't go too far wrong.

On the plus side at least you can go home at night,but your boss has to listen to her giving out at home and in work.

Failing all that,keep your head down and try to find other employment if its really getting you down.