I live with someone diagnosed with PTSD. The diagnosis came after the sudden death of a family member. That family member would have been 21 today.
Living with someone with the disorder is definitely challenging. I have to be careful of what I say and how I say it. We don't watch network television anymore and seldom even watch movies together. When we're outside, we're kind of on edge because there is shooting all around us. When it happens, I'm usually left outside, while my wife runs inside. Sometimes, if I follow her in, I may find her crying, or I may not.
Do I have any advice? Honestly, no. I only know what I do in my situation because it's been five years in the making.
I'm sorry for your loss, and I would just say be there for your nephew in the best manner you possibly can.