Accept the possibility of this.
However, if this is true, do you love your wife any less? I would hope not.
Your insecurity and inquisition play a roll in this theatre: as you question your wife, you are more likely to commit acts against her that you would consider equal and justifiable to the presumed transgression. What you are experiencing now is called cognitive dissonance. You are considering making a move on your wife's cousin. You are having conflicting points of view on your own identity and that of your partner.
Does your wife know about your long kiss with her cousin? If she felt that she was unfairly treated by your own indulgences, she is more predisposed to seeking to settle the score in some way.
Does it matter if she slept with someone else? Especially if it were discreet and no one else knows? The answer will come down to how comfortable you are revealing your own secrets. Just as much as someone is drawn to settling the score after an unfair interaction, people are drawn to return kindness and genuineness when it is perceived.
Spinning the topic slightly, this is part of the reason why no one should be 100% good in a relationship. Most people won't trust it and they will assume you are satisfying your inherent natures (sexual and otherwise) behind a facade of spoken intentions. You will find that some people are more at ease when you feel comfortable saying whatever is on your mind, good or bad. If someone is going to leave you for doing something in accordance to your nature, are they really right for you?
Your mutually shared fantasies of the couple were an exploration of that facet of your comfort. You made each other feel more comfortable by admitting to internal desire. Right now, you may just be trying to reconfirm that the relationship has continued to be fair. You want a "piece of the cake" too if she has done something.
It's not a bad thing; it's human nature. Show her the love she deserves as your wife, and follow what your heart feels. If you feel angry, let yourself be angry. Just don't do anything you feel you would regret (look into epicureanism versus hedonism).
There is a tit-for-tat rule when it comes to cheaters. If you want to learn something negative that someone has done against you, admit to something you have done against them. The moment they feel that you won't have the highground after an admittance, they are more likely to be open and honest about the subject. A second trick is to feign indifference, posing that your interest is purely for the sake of curiosity and because you are trying to "find out what makes them happy and give it to them." If there are no negative emotions associated with admitting something, there is less resistance to doing so.
A word of warning: the first version of the story that any cheater admits to is a downplay or deception of what actually happened. Volume and quality are often downplayed for the sake of trying to salvage their relationship with you. You need to take time to break down to the truth --- or accept the possibility that you will never have the full truth.
No one likes a pushover, and no one likes a whiner. But don't be afraid of the fact that sometimes we all are one.
She does care about you.