Die verwechselten Töchter [literally: The Exchanged Daughters] is an epistolary novel by Maria Anna Sager (also written as Maria Anna Sagar).
Finished on: 26.4.2019
[You can read it here in German.]

Plot:
In a rather poor neighborhood, two girls are born at almost exactly the same time, and both are called Klara. Their mothers are fast friends, and the two girls grow up inseparable and often indistinguishable. When the mother of the older Klara is called away by circumstances to acquire a more affluent position, both Klaras remain with the mother of the younger Klara. When the older Klara’s mother calls for her daughter a few years later, the younger Klara’s mother hopes to find a better life for her daughter and sends the younger Klara in the older Klara’s stead – a decision that causes troubles for all of them.

Die verwechselten Töchter is an almost forgotten classic of Austrian literature, one of the first (epistolary) novels by a woman to be published at all in German. And it is still a very good read that I can absolutely recommend.