Just be strong for your brother.
Acknowledge his concerns, get him to open up about how he feels when he's ready, empathise, hold him, hug him.
If he is feeling like he has to be strong for his daughter and her mother maybe it will be good for him to know that you are there for him to say what he is feeling without being concerned about feeling foolish or inept himself, because how do you express grief when you are mostly worried about how your child feels freely without feeling selfish?
Be open with him, call him and ask him how he is and let him know you are thinking about him, just like you, he might be feeling at a loss as to how to approach his daughter...if he can use you as a sounding board to get out his feelings on ..to rant to..it may help him to find the strength and words to comfort those nearest to him at the moment.
I'm really sorry to hear your news....I'll pray for a miracle anyway and I hope you all find something to smile about together again soon.
I think you just need to be completely non-judgemental and to remind him that you are there for him always.