Dbh,
I am so sorry. My brother committed suicide three years ago and I still find myself asking those same questions. What could I have done? How did I not know?
I don't know that there is a way to avoid those feelings in these situations. However, I agree with DreamsofSerenity. Go to the support group. I found one in my area and parked outside but never went in. Since I never went I can't promise it will help, but I know that the way I dealt with my grief was not the best way. I sunk further into my drinking and isolation and deeper into an unhealthy relationship where I was being given valium to "help me deal with it" even years later. Ultimately, my drinking and pill popping cost me my job because I didn't deal with my grief in a constructive way.
Since becoming unemployed, I have had LOTS of time to think about things. I am finally realizing that I am not to blame for what my brother chose to do. He was depressed throughout his whole life and self medicated with opiates to cope. There was nothing I could have done to prevent that anymore than anyone else could have prevented my drinking becoming a problem. People make their own choices and some of them hurt worse than others.
Try to forgive your sister and realize that she had gotten to a place where she simply saw no other way out. That is hard to do when you see the situation she has left her son and your family in, but it is still true.
Again, I encourage you to seek support for your grief. Each case is different, but suicide forms an unfortunate "bond" between those of us who have been left behind. I think you may find that you will feel more in common with those in the support group than you believe you will. I wish I had chosen to seek help for myself.
You will be in my prayers.