Strictly logically speaking, this is consistent with it being genetic + random chance (where the odds are determined only by your genes) - meaning no identifiable environmental factors. Not saying it is (nor would I know).
Were these identicals raised apart, or together? Are there enough of each to see a difference in rates between those raised together and apart?
Whatever the triggers are, I can't help but think that they're subtle and unmemorable.
This seems to be pretty compelling observations for various disorders like Alzheimer's, various autoimmune conditions, etc. being caused or exacerbated by previous infections.
It was the case for me. I apparently have the genes for a semi-rare autoimmune condition (Guttate Psoriasis) that was triggered after a bout of some kind of viral illness. Typically, step throat is the trigger for many with the disorder, but that was not the case for me.
With siblings it is likely both generics and environment are similar. In terms of trauma and episode triggering events, imagine the stress of living with a large family where several are schizophrenic.