I really wonder who will live longer, Olivia or Fontaine. I read somewhere that "de Havilland proactively wanted their mother (who was suffering from cancer) to be treated surgically, at the advanced age of 88, which Joan apparently did not think was a good idea."
Now, could this possibly be an indication that Joan Fontaine would refuse medical surgery if such a situation arose for her? And vice versa for Olivia? Maybe the latter one would have an advantage in that aspect and live longer, while the former would value life quality over quantity?
Another thing to consider...their feud stemmed from Fontaine sensing that Olivia was their mother's favourite child. Jealousy is a negative emotion, and negative emotions are considered to be counterproductive towards longevity. Would this give Olivia yet another advantage? Olivia doesn't have much of a reason to be jealous of her little sister, because she had a much more successful career (objectively speaking).