Not all I believe. The manuscripts that were in possession of Chopin's sister and Fontana were all published a few years after his death. Chopin had the habit of composing little works and gifting them to his pupils, acquaintances, friends, and patrons. Some were published by the recipients or Chopin (Mazurka in a minor a la Emile Gallaird) and others remained in the private possession of its recipients and their estate. Chopin would sometimes give them a handwritten copy of his (leading to several estates owning the same composition) or give them an original manuscript, he wasn't entirely consistent.
For example a prelude in ab published posthumously "presto con leggierezza" was originally a gift to Piere Wolf and the piece remained in the possession of his estate for decades. They had the only existing copy of the work. In the case of waltz no. 19 in a minor, the handwritten copy and original draft ended up in the hands of different estates, so Fontana and Chopin's sister were of unaware of its existence. Chopin handing out his unpublished works like candy to friends and stuff is the main reason why we still have Chopin pieces being published almost 200 years after his birth!
Chopin had a close relationship with the Rothschild family, Betty de Rothschild (Charlotte's mother) was one of his first pupils in Paris back in 1832 and also a patron of Chopin. This led to him forming relationships with other members of their family Nathaniel and Charlotte de Rothschild, Charlotte later became Chopin's pupil in 1841 (several of his pieces are dedicated to her, for example op. 52) or so. Since there are 3 draft versions of the c minor Nocturne available, I wouldn't find it hard to believe an extra one ended up in the possession of the Rothschild estate. Charlotte probably believed she had the only existing copy, since it wasn't published by Fontana.
Since Charlotte wasn't an accomplished musician, her publication of the piece under
her name didn't circulate well and was lost in history. So when the nocturne was published in 1938, the source was unrelated to Charlotte. Charlotte's "compositions" were only recently discovered, this decade I believe, by a Chopin enthusiast - it was apparently collecting dust in some old section of a library.
https://imslp.org/wiki/4_Pi%C3%A8ces_pour_piano_(Rothschild,_Charlotte_de)
You can his letters here:
https://archive.org/details/chopinsletters00chop