Sister, I am very sorry for your loss. I understand that feeling of wanting and needing answers. My own beloved brother died five years ago of kidney cancer, and to this day I have anxious nights sometimes, tossing and turning, worrying about all the unanswered questions -- some about how he handled his illness, some about the medical care he received, some about the behavior of family members before and after his death.
He was only 47 and it was 90 days from diagnosis until his death -- it seemed very sudden to me, but of course others like yourself have an even more shocking time-line to deal with.
I am telling you this to help you see that I have tremendous compassion for what you are experiencing and your sense of urgency to get answers. I do understand. However, I think it might be better for you in the long-run if you can focus on grieving and letting go now. There's nothing you can do to bring him back, and whatever he chose to do or not do, you'll never be able to speak with him about it or understand it, really. If you allow yourself to be haunted by these unanswered questions, will it really help you heal? I fear not.
Grief is a process that takes a long time. For me, the really acute grief lasted about eighteen months and it was almost four years before I could think about him with happy memories and not instantly fall into sobbing. We were really close (more like twins, in some ways, than just brother and sister.) I miss him terribly. But I also know that he would not want me obsessing about his death or staying mired in misery. I'm sure that your brother wouldn't want you to compound your grief with suspicions, anxieties and doubts, either.
If you can get into a grief support group or work with a grief counselor, they might help you put your unanswered questions into perspective. In the meantime, here is a (((hug))) from a stranger who wishes you well.