Realistically, I wouldn't have to tell my 14yo anything, because the likelihood of your scenario ever happening is so vanishingly small as to be nonsensical. That's a genuine "well established fact".
So, let's suppose a slightly more realistic scenario. Let's say there's a massive derailing accident on the monorail and my 8yo dies while his 14yo sibling survives. In that scenario I would comfort my 14yo, reassuring them that they were not to blame in any way and no one could have done any more to protect their younger sibling than they did. And then I'd get us all into therapy and look into whether or not a lawsuit is justified.
Even MORE realistic... my 9 and 11 year old once went to get pizza at a place five blocks away from our home. While standing in line, my 9yo (undiagnosed hypoglycemic) suddenly passed out, fell over backward and hit his head. Bystanders immediately called 911, while my 11yo, remembering what we'd roleplayed many times in the past, latched on to a friendly looking female college student. My daughter borrowed the girl's phone and called me. I ran down to the pizza place, and found her standing on the corner with her young lady, waiting for me. Meanwhile my son was in an ambulance being checked over.
My son was fine. But my daughter, a few times, expressed that she felt guilty because she saw him fall, but didn't catch him before he could hit his head. I told her, as many times as she needed to hear it, that she'd done amazingly well and had nothing to feel bad about. No one expected her to catch her brother.
So did I feel guilty for letting my kids go off alone to the pizza place? Nope! It's a reasonable thing for a 9 and 11yo to do in the middle of summer, in my neighbourhood. Did I feel any guilt because my daughter felt a little bit of "emotional pain"? Nope! I simply helped her work through it.
Life happens. We deal. That's what growing up is all about.