I'm so sorry ! Please don't torture yourself with what you could or should have done different. Remember the good times, and forget the rest.
Think of his life and what you enjoyed together. The day he died is one to forget. It's one day , and there were many better ones, those are what you should hold onto. Forget the day he died and hold onto your brother. Make some memories with him and enjoy life. Life is precious, enjoy it, anything else is a waste.
My thoughts and prayers with you.
Bryan