How sad. My cousin is losing her mind right now from something similar. She's been living with him for 3 years, and last Saturday he woke up, walked into their living room where she was watching TV, turned the TV off and said 'we need to talk'. Within 2 hours, he was gone. She's heartbroken and shocked. They had just gotten home from a vacation, and he was saying he loved her the day before.
How people can be so cold, I don't understand. What ever happened to working on things instead of just walking out the door?
I hope there is no one else involved with your SIL. If there isn't, then as awful as it is, I have to say that if she wasn't happy, she did what we all ask - why didn't you just leave?
I'm sorry your brother is hurting. I'm thankful that someone is so much pain has you as a sister to help him through this.