Story of Incest Nothing more Nothing less. (Spoilers)...
Edgar Hart no doubt is the main corrupter of all three women....as a result all three are too ashamed to show affection that could remotely hint that they are related and in doing so they manage to convince themselves that they are strangers...its just like if you lie about something too many times you end up believing it yourself....but at times of peril the blood affection buried deep inside, subconsciously emerges....Examples...reaction of both Willie and Millie as they see Pinky drowning...reaction of Pinky as she sees her mother (Millie) with Edgar....reaction of Millie as she finds out that Willie is about to give birth....and the first thing Millie says at the birth without even checking that the baby is alive or not that "Its a boy" inadvertently a subconscious check to make sure that Edgar has no more women to abuse......
Pinky behavior from start to end holds the key as the initial adoration for Millie a subconscious effort to please her mother...the transition in Pinky behavior after she comes out of coma is again a subconscious attempt at hostility for her mother for failing at various moral aspects and not to be the icon of perfection that she carved out....
The end reflects that after the death of the corrupter things can more or less go back to normalcy.