Hi, I'd like to help you with your question.
Affairs are always a shock. It does not matter if they happened yesterday or 16 years ago. Infidelity shakes the foundation of a marriage and removes the trust you believed was there between you and your partner.
There is nothing wrong with how you feel. You have a right to feel upset about this. Your wife and your brother, two people you trusted, betrayed you. You are the wronged party here. Yes, letting your wife and brother know that you know is fine. There is no way to heal unless the situation can be brought out into the open and worked on.
In order to heal, your wife and your brother need to regain your trust. Being that this happened many years ago, it probably has been over between them for a while. But that does not mean it should be forgotten. Your perception of your wife and brother have changed with the knowledge of the affair. That means they need to work with you to rebuild your relationships. Both of them need to admit to the affair and be remorseful. If they are not sorry for what they did and/or try to blame it on you, then they will make it almost impossible to repair their relationships with you.
Counseling is probably the best way to help all of you work this out. To find a therapist, talk with your doctor for a referral. Or, if you attend church, your pastor may be able to help. Pastors are often trained to help married couples through such issues. Or you can search on line at http://therapists.psychologytoday.com/rms/.
You can also work on this issue at home. There are many resources out there to help. Here are a few to get you started:
Infidelity: A Survival Guide by Don-David Lusterman
Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity by Shirley P. Glass and Jean Coppock Staeheli
How to Help Your Spouse Heal From Your Affair: A Compact Manual for the Unfaithful by Linda J. MacDonald M.S. - for your wife
You can find these books on Amazon.com or your local library may have them for you.
Remember, you have the right to feel sad, angry and upset about the affair. But with some work on your wife's part, you can heal your marriage and even have a stronger relationship than before.
I hope this has helped,