'The familial relation may be more likely to be associated environmental conditions. '
I take your point, but my only problem with the environmental angle is that I would expect a 'cluster' of cases affecting others in the same area. I'm not aware of any epidemiological reports or studies that confirm such a distribution of incidence.
On the other hand, if the entire family were, for example, accidentally sprayed by pesticide/s as children on a day out (the family lived fairly close to a rural environment), then conceiveably we would have a prime suspect that could explain the high rate of incidence within one family.
At the very least, I think the jury is still out re. probable cause, but I'm more concerned with finding a cure for them than worrying too much about how they got to this state.