Accidental drug overdose is definitely not suicide.
when people use drugs excessively, yes it is a form of suicide because they don't care if they live or if they die. if someone dies after using for the first time (and that does happen) I can see your point. but for anyone to get deep into drug use or at all consider it, something is missing from their life inside of them and they are looking to fill it with something that they think will make them feel better. they are not looking to hurt or punish anyone they love. the duality of wanting to punish themselves but still seek some sort of comfort is what is there. they may not say it out loud but in the back of their mind they know the possibility is there and for many the thought of being at peace is what they are looking for however they can get it. It doesn't make sense I know. Thats why addiction is so horrible. It distorts reality for the user. That said, I cannot speak for your brother. I wish he had never heard of drugs.
I am sorry for your loss.