Chopin nocturne C#-, historical background?

[emmdoubleew]

I'm wondering why this piece is posthumous.

I know that "op. 72" was posthumously published against Chopin's wishes and was the first nocturne he wrote, but no idea about this one.

My edition (by william palmer from the original sources) says: "This piece was composed in 1830. It was published without opus number by Leitgeber, a Polish firm, in 1875. At the top of the first edition, these words (in Polish) appear:
'For my sister Louise to play, before she practices my second Concerto'"

Does anyone know more?