Thank you for writing in. This sounds like a very complicated situation fraught with the potential for significant consequences for many people, some of whom you love dearly. I’m curious why your mother chose to share this tragic portion of your family history with you now, after all these years. Is she proposing to do something about it? Is she trying to unburden herself after carrying this disturbing knowledge alone for so many years? Does she have reason to believe that your cousin is abusing his children and/or members of the football team? Is there some other motivation for the disclosure? If you haven’t already done so, it might useful to talk to your mom about why she shared this with you at this time. It is possible that her answers might help you decide what you will do with this information. For example, if your mother thinks that someone is presently in danger, then there would be ample reason to take this information to the police immediately.
If you do decide to go to the police and open an investigation, they would probably want to speak to your family, especially your sister and brother who were victimized by your cousin. If this is the course of action you choose, it would be best for your siblings to hear this from you and your mother, and possibly also a therapist in a safe, therapeutic setting. It would be unfortunate for your siblings to first learn about this from police investigators.
I would also suggest that you schedule a consultation with a therapist who has a strong specialization in work with childhood sexual abuse. Someone with an extensive background in this area would be very well equipped to meet with you and your mother, listen to the history you present, ask additional questions, and ultimately make some recommendations for developing an action plan that will serve everyone as best as possible. Consulting with a therapist in this capacity might also prove to be quite therapeutic for you. In reading your question, I can almost feel the anxiety you are likely experiencing. You might find it quite helpful to have the support of a therapist during this time.