According to my husband of 35 years, men don't leave women (unless there is massive abuse or infidelity) unless there is another woman. I suspect that's why your sister's husband doesn't want to talk about it or go to counseling. And I could be wrong, but probably am not. Still, it's terrible that a life-long relationship that has produced children is falling apart in this way. It's very sad for sure.

I hope that she will indeed find herself and thrive eventually, but she's absolutely going to need you and others who have her back in the meantime.