

And Vida's mother may have been an intelligent woman for all we know, so it is certainly possible that Vida's intelligence and self-motivation was inherited. While Charlie was certainly no genius, Isabelle was rather clever, in her own way. Vida is the result of a union between Charlie and some unknown woman. This is why they were never able to self-educate successfully.

So, the twins are impaired (what used to be referred to, in bygone days, as an "idiot child"). The twins are the result of an incestuous union between Charlie and Isabelle, both of whom were "odd" at best and none-too-bright at the very least. I've read this book several times, but it's been a year or so since I last read it, so I may have some facts slightly wrong, but here is what I think:
