So sorry about all the stress and loss your family has experienced. This is obviously a tough diagnosis and will impact the entire family.
Clearly there will be many crises, many ups and downs as this progresses, but even still, it's possible that the toughest time for your brother is right now, after having just received this heartbreaking news. The adjustments he must make - cognitive, emotional, physical - to accommodate this change in expectations and in the family reality are huge. Everyone must give up the dreams and hopes they had for this child, to make room for what is and what is in store.
But the human psyche is an amazingly adaptive and ingenious thing to behold. Success will be measured in the daily gains made by her and everyone else, in coping and solving problems as they come up; and in the family pulling together to learn and grow and support one another as each member goes through this in his or her own way. The healthy kids will need an outlet to talk about this and negotiate their way through this too. Not easy. But it can be a profoundly strengthening experience for everyone with the right kind of support.
I hope your brother seeks out help for himself and the family. His wife (who carries the gene), will need help too. This is a tough place to be, psychologically. Even though it's irrational, she may feel that she’s to blame. There are support groups for his disease, and family counseling - and perhaps couple's counseling - will be important resources, too. And of course he has you and his other siblings.
All that said, I suspect the starter imagery that might be useful at this time would be Heathful Sleep. If it looks like he's responding well to this imagery, depending on his state of mind, he might do well with Anger & Forgiveness, Ease Grief or even Healing Trauma. But I wouldn't start there… I'd get him acquainted with imagery slowly and simply, by trying to help him sleep better with it.
I hope this helps. Please write if you have any questions about anything I've mentioned that isn't clear or complete.