Copyright laws (and the durations in particular) vary by jurisdiction (as influenced and occasionally governed by international treaty). US copyright law is very convoluted mainly because they have had (for various periods) a distinction between published and unpublished works, and for those with and without copyright notices applied.
In particular, the ‘1923 rule’ (which only applies to published works, anyway) is a purely US device.
Dutch law (see the Copyright Act 1912, art.37) holds that copyright subsists until 70 years after the authors death (actually, 70 years after the 1st Jan following their death). Of course, the owner of this intellectual property can grant whatever licence they like during that period (as long as it does not exceed their own rights), including waiving copyright entirely.
Cissy died in 1948. Assuming no early waiver exists, that means that unless there has been regulatory intervention on her estate’s behalf, her books will remain protected until Jan 1st, 2019. Such intervention does occasionally happen… the (UK) copyright to Peter Pan subsists in perpetuity as a result of a specific piece of English legislation. JM Barrie left the rights to the book to the children’s hospital Great Ormond Street, who successfully petitioned to have the rights continue as the royalties provide significant funds to support this charitable hospital.
Of course, that’s just applicable in the Netherlands, although international treaties mean that those provisions are well reflected and respected in most other jurisdictions, so publication for any derivative works would be restricted to the handful of countries that don’t play nice with others. Ironically, the US is one of them.
GOSH, for example, has pursued several lawsuits against US parties who try to apply the 1923 rule to Peter Pan (written in 1911, originally out of the normal position of life+50 in 1987, later extended to 2007 by the EU’s move to life+70) while it was still subject to the in the normal copyright rules. GOSH lost some of those cases, if I recall correctly, but not without costing both sides a lot of time and money on each occasion. The confusion stems in part from the Berne Convention which basically says - gross oversimplification alert - that (a) parties should apply the local rules for the publication, not for the creation, and (b) those local rules should have at least life+50 in them. The US is happy to apply the local rules, but doesn’t always apply life+50.
As JT points out, copyright law - especially cross jurisdiction - is both complicated and ever changing. When I first started studying the subject, UK law only protected for 50 years after the author’s death… it’s now 70 years (a fact I’ve ben known to forget from time to time 
). The US is even more changeable as regulators make adjustments to reflect the changing reality of life. People are living longer (as are their inheritants) and producing copies of artworks is substantially easier (and essentially free) compared to the position in the early 1920s.
The notion of public domain works was designed to protect innovation, and preserve art… but it’s actually one of the least effective ways of doing so in the modern era. I’m with Disney on this one. The argument of whether to extend protection for Snow White essentially boils down to the question ‘who would I rather be allowed to make money from this?’. Personally, I’d prefer it was the people responsible to creating the art in the first place, their descendants, and the people who have carefully protected and cultivated that work for so long, as well as invested time and money in ensuring that it is still something that people actually care about watching so many decades later. The alternative is that the exact same money goes to… anyone who cares to press Ctrl+C and then Ctrl+V.
The end result? Pride and Prejudice becomes Pride and Prejudice and Zombies. Snow White and the Seven Dwarfs becomes whatever perverted fantasy you can splice into it on iMovie that you’d probably prefer your kids didn’t watch(*). Which approach better preserves the artwork? I know which I’d prefer.
* - to quote The New Statesman(**) “I’m pretty sure there isn’t a dwarf called ‘Horny’.”
** - the TV series starring Rik Mayall, not the periodical.
