I really cannot add much to the wonderful responses you have received.
I worked for three years with drug addicts and alcoholics, and I can definitely say that your brother probably DID take the combination which resulted in his death ACCIDENTALLY.
The minds of those who are trapped in this illness are never clear, and deep depression is always a factor which is constantly present. In that state, people often do not know what they are doing. They forget how much they have taken, and lose touch with reality.
I feel so deeply for your distress. THIS IS NOT YOUR FAULT!
I think I can also go as far as to say that it is not his fault either. He was a victim of his illness, and it is an illness that is terribly hard to control.
You need feel no shame, guilt or embarrassment, no more than if your brother had died of cancer. He was very ill, and now he is free - however it happened.
Please do not beat yourself up over it, and do not allow others to sit in judgment of you or of him. THEY are not the ones who had to live with the pain, and no one can ever know the extent of another person's pain.
"Talk" to your brother in your heart, and tell him all that you feel. Tell him that you are angry BECAUSE you love him. I believe that somehow, somewhere, he will hear, and will know, and will be at peace. And Jennifer, you can be at peace too.
I am sending you all my love and I will keep you in my thoughts as you move on into the future.