"Kateřina's mother was not even sixty when she died."

Translation: Kateřinině matce nebylo ani šedesát, když umřela.

[yession]

Who is "she" here, Katerina's mother or Katerina?

[AdrianTomin]

As a native English speaker reading this, they are talking about the mother; unless they specify otherwise.

[LaynePierc1]

what is wrong with including let, "sedesat let"?

[VladaFu]

Nothing. If you want to know what is wrong with your answer, you have to tell the complete sentence.