I am so very sorry you are going through this terrible time. I understand how you are feeling..............when our son died in a car crash I kept looking for reasons why and wondering what I might have been able to do to have prevented the accident..............the answer of course was nothing.
You have been a wonderful and loving sister and please do not let the mumbo jumble talk of brain misfiring cause you to think you had anything to do with your sister's death. You do not have that kind of power my dear friend, some things are just out of our hands.
I am sorry your Mom was not able to communicate with you and tell you the findings, losing a loved one is horrible and dealing with all the family dynamics is difficult for many of us.
Coping with an illness as serious schizophrenia is very challenging for families and the person with this disorder. You made all your decisions out of love for your sister so please do not allow the "stinkin thinkin" to creep in. Kick it to the curb and stay in the present moment.
Remember you are grieving your sister's death and grief spans a huge gambit on the emotional spectrum. Let yourself cry for your sister but believe that she knows how much you loved her and you would never have done anything to harm her.
My prayers and hugs are with you.
I hope you make it through the day and remember my email is always open.
I wish you peace,