There really are no words in times like these. My heart goes out to you, your brother A, to D and his family, and everyone else affected by this tragedy. It's unspeakably horrific and incomprehensibly sad.
You are right that despite all efforts, your yoga practice cannot beam enough healing across the world to save D... But it can support and sustain you in dark, difficult times. Your post, written through a sheath of grief, reminds me of the words of B.K.S. Iyengar:
"Yoga teaches us to cure what need not be endured and endure what cannot be cured."
or, at the very least - try.