Don't beat yourself up about your friend, I was in the same situation some years ago. The fact that she is reaching out to you now tells me that she is a good friend and understands, friends like that are pure gold.
You are not useless to anyone, you are struggling with the hardest experiences that anyone can go through and grieving the loss of your beloved brother. You are also reaching out for help on this forum and that is braver than you know.
Keep remembering those times of love and laughter, if only we had a time machine and could go back and burn those times in to our memories.
Take care of yourself today x