Here's my take on your posting. I think you did the right thing in contacting BIL about the situation w/his son. The info you provided will help and his son a great deal.
Now about the other situation concerning your BIL and cheating on his wife. There is nothing you can say to this man about his situation because it is none of your business. In fact, you've got a full plate dealing w/your own situation. It's not your circus. I think what you said to your wife was spot on and if she's got issues w/him, then it's up to her to decide how to deal w/it...but you need to stay out of this situation.
I think you handled it well. Remember...she fired you as her husband. She needs to grow up and take care of her own circus. You are no longer part of that circus.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
