Hi, I'd like to help you with your question.
Your sister has put you in a bad position. This is not an easy situation to navigate. However, this is your sister's marriage and it is up to her to tell her husband about the affair. If you try to get in between, you will become the target and may lose both relationships in the process. And you may even be blamed by your family for a break up, if it should occur.
It is best to let your brother in law know that you and your sister had a falling out but that you cannot provide an explanation as to why. Direct him to speak with your sister to find out her side of it. Then let your sister know what you told her husband. Explain that you no longer want to be in the middle and you will direct all inquires from her husband her way. She needs to be responsible for her actions, including those outside of her marriage.
If you are willing to try to save your relationship with your sister, consider going to counseling together. Your sister appears to have some issues and this has affected not only her marriage, but your relationship as well. Therapy could help you both come to terms with what happened and maybe help your sister work through her issues. To find a therapist, ask your doctor for a referral. Or search on line at http://therapists.psychologytoday.com/rms/.
I hope this has helped you,Kate