My sister-in-law's husband confided in me last night that SIL has been depressed for the past 4 years and at times suicidal. I was horrified to hear this. I urged him to contact a doctor and professional assistance and he said he had. He said the cause of it is a family rift between my SIL and her younger sister, which he thinks is a recent event however I've been told by her mother that there has been bad feeling between them from childhood (intense rivalry academically and around image, etc.).
The whole thing is compounded by the fact that their mother stirs the pot between them (favoritism towards the younger sister mostly) and has passed some of her own issues on to both of them (specifically image issues, weight, etc.). To top it all off, they're all incredibly stubborn and won't give in. It's been a mess for years and I purposefully have stayed out of it and neutral on all the issues so far.
I had the feeling that BIL was telling me as he wanted me to try to intervene between SIL and her sister, but I'm reluctant to get involved knowing the complexities. However, I feel so bad for SIL. Am I right to stay out of it, or in the circumstances should I say/do something?