Hi warpig
First of all, my sincere sympathies for the situation in which you find yourself.
As you can by the speed and depth of responses on here, there are a number of AB'ers with experiencec of depression and suicide, wshich at least will let you know you are not alone.
First things first - your SIL's situation is NOT your responsibility, so don;t torture yourself looking for the 'right' thing to do or say.
Follow your insticts.
Be there for her when she comes to you, leave her alone when she doesn't.
If you can't think if the 'right' thing to say, say nothing - you are doing the best thing by being company for her while she gets her thoughts in order.
Try to encourage her to talk about what is troubling her - to you, or to someone like The Samaritans, who have vast experience, and can be there at times when you are occupioed with your own family.
Share with us - we aill support you when you need it.
Remember, neither the cause or the actions of your SIL's behaviour are down to you, so no worrying about 'seeing the signs' or 'doing more' - none of us have a script for this, we learn as we go.
Don't get so wrapped up in this that you harm yourself, even a little - you must maintain your own equilibrium through this.
Don't worry - it will workj out for the best in the end.
Thinking of you.
A x