W tej księgarni szukam powieści, której autora bardzo lubi moja siostra.

Because w means in here, and when w indicates location, Polish uses the locative case.

• księgarnia → księgarni in the locative singular
• ta → tej to match the noun

So:

• w tej księgarni = in this bookshop

Because the verb szukać normally takes the genitive case in Polish.

So:

• dictionary form: powieść
• genitive singular: powieści

That is why you get:

• szukam powieści

This is something learners often just have to memorize with the verb:

• szukać kogoś / czegoś = to look for someone / something

By itself, powieści is ambiguous. It can be:

• genitive singular of powieść
• or nominative/accusative plural

But in this sentence it is singular.

The clearest clue is której, which is feminine singular and refers back to powieści. So the sentence is talking about one novel, not several.

If it were plural, you would expect something like:

• powieści, których autorów...

Której is a form of the relative pronoun która / który / które.

It refers back to powieści, so it must match that noun in:

• gender: feminine
• number: singular

But its case is determined by its role inside the relative clause, not by the main sentence.

Here it means something like:

• the novel whose author...
• literally: the novel, the author of which...

That is why Polish uses której here rather than która.

Because której does not depend on szukam. It belongs to the relative clause.

Main clause:

• szukam powieści = I am looking for a novel

Relative clause:

• której autora bardzo lubi moja siostra = whose author my sister likes very much

Inside that relative clause, której shows possession/connection: author of the novel. So it needs the form used for of which / whose, not the form for a direct object.

So:

• którą would be wrong here
• której is correct

Because autora is the direct object of lubi.

The verb lubić takes the accusative case:

• lubić kogo? co?

For masculine animate nouns, the accusative singular often looks the same as the genitive singular.

So:

• autor = nominative
• autora = accusative singular here

That is why:

• moja siostra lubi autora = my sister likes the author

It is accusative here, even though it looks like genitive.

Why the confusion? Because masculine animate nouns in Polish often have:

• genitive singular = autora
• accusative singular = autora

In this sentence, autora is the thing being liked, so grammatically it is the accusative object of lubi.

Because Polish word order is much more flexible than English word order.

The basic grammar is still clear because the case endings show who is doing what:

• moja siostra is nominative, so it is the subject
• autora is accusative, so it is the object

Putting moja siostra at the end gives it a bit of focus, as if the speaker is highlighting who likes the author.

Other orders are possible too, for example:

• ..., której autora moja siostra bardzo lubi.

That sounds a bit more neutral. The original version is perfectly natural.

Because in Polish, relative clauses introduced by który are normally separated with a comma.

So:

• powieści, której autora...

This is different from English, where that/which/who clauses are not always preceded by a comma. Polish uses the comma much more regularly in this kind of structure.

A very literal breakdown is:

• W tej księgarni = in this bookshop
• szukam powieści = I am looking for a novel
• której autora = whose author / the author of which
• bardzo lubi = likes very much
• moja siostra = my sister

So the grammar is built like this:

• I am looking for [a novel]
• [whose author] my sister likes very much

That kind of nested structure is common in Polish and is worth getting used to.