Joe you mention that you've seen this "look" before in her, so to speak. Are you suggesting that this isn't the first time?
You also use the words for the Cousin as "a boy" and your wife is 44. So is he a younger guy in his early 20's? Could she be feeling old, used by date, have a need to feel young again?
You say you work a lot, cook a lot, do chores. Do go on holidays, party, make love.
It doesn't sound like a boring life but she may be seeking "adventure".
You can't in my opinion put things in the past without facing the present to work on a future.
I think you need to see a counsellor as well, sit down and work out her reasoning for doing it, don't just let it slip and go quiet, it will eat at you.
If she can open up and explain her reasons/actions and be honest with you, perhaps through counselling you can work at what is missing in your lives, I am sure that if you sat back and thought about it, there are things missing in yours as well, from her.