Absolutely!! I have been amazed to learn over the years from friends (and here on SS) that the issues in our family that I used to think were not common, are in fact more common that I would have imagined. Like me, some friends have felt their family issues were uncommon, and tended to keep the issues to themselves.
Once I opened up about the things we had to deal with, and my friends would tell me their stories, I started to realise how big a problem mental illness really is. I feel so much more "normal" now that I realise our family was not unusual at all, and our issues are shared by so many more than I could ever have guessed.
The other thing is that this SIL's mum will not discuss the subject, insisting that she "is fine now", just before she crashes once again. The denial is frustrating, like she is trying to hide the problem from her own family.