There is a nice article in The Independent about the sibling rivalry between Joan Fontaine and Olivia de Havilland, both in their nineties now.
I have become a huge fan of Joan Fontaine after watching Letter from an Unknown Woman. She is great in Rebecca and Suspicion too but Letter is in a totally different league. I am not that familiar with her elder sister. The only film of hers that I have seen is William Wyler's The Heiress which I didn't like that much. She is actually quite good in it, in fact she won her second Oscar for this role but the film itself is overly simplistic and even manipulative. I understand it is not fair to criticise a melodrama for being melodramatic, but unlike in a good melodrama the exaggerations, the emotionalism, arbitrary plot reversals etc are not used to challenge viewer's expectations but rather to reinforce pre-existing ideas. All the actors are quite good though, and the film is certainly worth watching. I need to read Henry James' novel Washington Square too, on which it is based.