My sister committed suicide from a drug/alcohol overdose. I know it depends on the drug (s) & amount of alcohol. Would this been a painful death?

Usually not. People who have witnessed such deaths describe them as peaceful. I'm very sorry about your sister and whatever it was that caused her to decide life wasn't worth living. Our minds play tricks on us, and each of us has times when we are not ourselves. I hope you'll cherish the memories that you had together, and live your own life richly and well.
For obvious reasons. We don't wish to opine on means of suicide in a public forum. This question would best be asked of the coroner who consulted, or your own personal physician. Every suicide is painful, if not for the victim, then to varying degrees by the survivors. Best wishes.