So Sorry for your loss, all you can do is be there for your brother when he is ready to share his feelings, just let him know you are there to support him on his terms. I was 10 when my best friend and cousin died she was 11. The best thing anyone did for me was not to pressure me to talk about it. People grieve on their own terms even young people.
In my thoughts and prayers, I hope you all can find some peace of mind and soul<3