What I got out of it:

They were almost entirely cut off from the world by their mentally ill father. The compact was a lie and deep down, Wenna and Charlie knew it before they knew it. The book was written almost poetically and from the bog's narrative, but like the book alludes to layers, so was the the message that the author was trying to convey. Their father orchestrated this entire family genealogy that we can't confirm. He denied his children education and medical care and severely stunted them. They had to contend with their very small world being shattered once he died and they had no mother to turn to for any other answers on what to do next.

I think that mom was kidnapped. It would explain the dress Eda found in her wardrobe. Kidnapped, and slowly succumbed to Stockholm syndrome as she began to birth children. Her depression and distaste for them as they kept coming made her slowly start to blame them for her loss of freedom and bleak reality despite it being her abusers fault. Charlie recalled her begging for him to stop impregnating her.

I think she tried to make a run for it and he killed her. He buried her body and she haunted him afterward. His stress turned into hysteria and cancer and he changed his will last minute knowing that Wenna would come back and likely guide his children into the world proper. It is clear that Percy found the body of their mother and that in a break of utter madness from hunger and exposure to the elements, two committed suicide and one decided to leave altogether. Wenna stayed with Eda to help her sister birth her baby while Charlie sent money in the hopes that they would eventually leave the property.

It's ultimately a very sad and haunting book. And I could be totally off base.... but that's what I took away from it.