Is that true, or was it a lack of education? The twins never went to school, what education they had would have been rather piecemeal.
It wasn’t uncommon in those days (and even later days) to automatically physically disabled babies as “mentally retarded” too, based on little or no evidence other than being physically different. You start to wonder if it’s a self-fulfilling prophecy, as back in those days less effort was expended on educating those presumed unable to keep up, as opposed to today when there’s a notion of putting more effort into people to bring them up to speed.