At the end of the book, Rae describes a very neat scenario that explains her understanding of the probably actions that occurred immediately before the house burned down, and the probable causes of those actions.
However, it's really all just speculation on her part.
Her grandfather William was the eternal boogeyman in her subconscious. He was always right there, cutting her down, calling her worth into question, being persistantly disapproving, and generally doing a number on her self-image.
Her great-uncle Desmond, on the other hand, has always been like a companion to her. He was right with her when she was defying William, disliking William, and generally disrespecting William. What a shame that she couldn't get away from William. Her later life would probably have been more pleasant. (But would she have been as amazing an artist without the spur of William's disapproval?) Desmond's journal supports her fellow-feeling about William.
Both William and Desmond were nuts. Desmond had the same issues with depression that Rae had. William was (apparently) a violent paranoid schitzophrenic. (OK, re-read all those chapter headings - that was William. Creepy.)
Desmond found peace and contentment before his death. William lived out his life as a bitter, angry, vindictive old man.
Rae had DNA. She had Desmond's bones. She had (probably) his and Lacy's hair. She has her own DNA.
In Rae's shoes, would you have run a DNA test to see if who her father's actual parents were? Would it have made a difference either way?
To DNA or not DNA... That is the question.