Steve, every person on this board is going to ask you 'so what?' and tell you to move on. And they are right. But I am curious what you are going to do with this information? Because in my mind (and in my situation) you can use this information in two ways:
1. Get sucked into the drama and codependency, think you may have a chance, try for the 5th (6th, 7th, 10th?) time to get her back
2. Use this to fuel you to the next level of detachment, she made this bed, she made this choice, let HER suffer the consequences.
You are a great person. A good man. So worthy of so much more than being Plan B.
It is somewhat cruel, though I am sure well-intentioned, of your SIL to share these details with you. She has her sister's best interest in mind. But she doesn't have your best interest in mind. Only you can do that. Choose number 2.