Yorkiegirl, how are things? I had another thought on this. One thing you can do is to check you have lots of contact details for your SIL - friends, relatives, neighbours, the name of her GP, the children's schoool, etc - so if there is any sort of crisis, you have the means of getting help quickly.
I don't know how you can go about this, but if possible, can you ring up some of her friends and neighbours who see her a lot, asking them to ring you if they are worried?
My friend changed her phone number on the advice of her therapist and stopped giving her new number to people as she slipped further and further into depression, cutting herself off, which made her depression and confusion even worse. I had to look through an old address book to find the phone number of her sister - lucky I had it.