Breakdown of Kannte die Frau die Königin und deren Schwester?
die Frau
the woman
und
and
kennen
to know (somebody/something)
die Schwester
the sister
Questions & Answers about Kannte die Frau die Königin und deren Schwester?
Why use deren instead of ihre here?
If the sentence used ihre Schwester, it would be ambiguous: is it the woman's sister or the queen's sister? Using the demonstrative possessive pronoun deren makes it crystal clear that we mean the queen's sister. Deren typically points back to the most recently mentioned noun or the object, rather than the subject (die Frau).
How do we know to use deren rather than dessen?
The choice depends entirely on the gender and number of the "possessor". Since the queen (die Königin) is feminine, we must use deren. If the sentence were about a king (der König, masculine), we would use dessen instead.
Would deren change its ending if the sister was in a different grammatical case?
No. Unlike normal possessive determiners (like ihre, ihren, ihrer) which change endings to match the noun being possessed, dessen and deren are fixed. Their form only depends on the owner (the queen). It stays deren regardless of the case of the sister.
Why is the verb kannte used instead of wusste for "did she know"?