I am so sorry you are in a bad place right now but please do not blame yourself for your sisters death. Heart disease is the leading cause of death in the United States, and its about twice as deadly for people with schizophrenia.
Your sister could have had a major arrhythmia such as ventricular fibrillation. Inconclusive cause of death means they don't know for sure one way or the other.ECT is one of the safest procedures done under a general anesthetic. The rate of death from ECT is less than one-tenth of one percent.
Please know that you did nothing wrong. I know it hurts to be told the autopsy results were inconclusive but try to remember your sister in a better time.
You were a good sister and you did your very best so now grieve in whatever way that works for you but remember you have your own angel watching over you.