Min søster er nysgerrig, så hun læser titlen igen, før hun åbner romanen.

In Danish, a possessive like min already makes the noun definite in meaning, so you normally do not add the definite ending too.

• min søster = my sister
• søsteren = the sister

So min søster is correct, while min søsteren would be wrong in standard Danish.

Danish possessives agree with the noun:

• min for common gender singular nouns
• mit for neuter singular nouns
• mine for plural nouns

søster is a common gender singular noun, so you use min.

Examples:

• min søster = my sister
• mit hus = my house
• mine bøger = my books

The -en is the Danish way of marking the definite form for many common-gender nouns.

• titel = title
• titlen = the title

Unlike English, Danish often puts the at the end of the noun instead of using a separate word.

The same thing happens with:

• roman = novel
• romanen = the novel

Because Danish usually expresses definiteness with an ending attached to the noun:

• bogen = the book
• titlen = the title
• romanen = the novel

A separate definite word like den/det is usually used only in special cases, for example with adjectives:

• romanen = the novel
• den interessante roman = the interesting novel

So in this sentence, titlen and romanen are perfectly normal definite forms.

Nysgerrig means curious.

Here it comes after er, so it works as a predicate adjective:

• Min søster er nysgerrig = My sister is curious

Danish adjectives do not change form in this position:

• Han er træt = He is tired
• Hun er nysgerrig = She is curious

So nysgerrig stays the same here.

Here så means so in the sense of therefore / as a result.

• Min søster er nysgerrig, så hun læser titlen igen
  = My sister is curious, so she reads the title again

Danish så can also mean then in other contexts, so the exact meaning depends on the sentence.

Here, it links cause and result:

• she is curious
• as a result, she reads the title again

Because så here is a coordinating conjunction meaning so, not an adverb placed at the front of an independent clause.

After a coordinating conjunction like og, men, or så, the next clause usually keeps normal main-clause order:

• hun læser = subject + verb

So:

• ..., så hun læser titlen igen is correct

If så were being used more like a fronted adverb in a new main clause, you could get inversion in some contexts, but that is not what is happening here.

Yes. Both are present tense forms.

• læser = reads / is reading
• åbner = opens / is opening

In Danish, the present tense often covers both the English simple present and present progressive, depending on context.

So hun læser titlen igen could mean:

• she reads the title again
• she is reading the title again

The broader context or translation decides which sounds most natural in English.

Igen means again, and its position here is very natural in Danish.

• hun læser titlen igen

A very common Danish word order is:

• subject + verb + object + adverb

So:

• hun = subject
• læser = verb
• titlen = object
• igen = adverb

Putting igen before the object would sound unnatural here.

Because før introduces a subordinate clause.

In Danish, subordinate clauses do not follow the normal main-clause verb-second pattern. Instead, they usually keep the order:

• conjunction + subject + verb

So:

• før hun åbner romanen = before she opens the novel

Compare:

Main clause:

• Hun åbner romanen = She opens the novel

Subordinate clause:

• før hun åbner romanen = before she opens the novel

That is why there is no inversion after før.

Here før means before and is being used as a conjunction, because it introduces a whole clause:

• før hun åbner romanen

If it were followed only by a noun phrase, it would function more like a preposition:

• før middagen = before dinner

So in this sentence, it is a conjunction because it introduces hun åbner romanen.

The commas mark clause boundaries.

• Min søster er nysgerrig, så hun læser titlen igen, før hun åbner romanen.

The first comma separates two main clauses:

• Min søster er nysgerrig
• så hun læser titlen igen

The second comma comes before the subordinate clause:

• før hun åbner romanen

Comma rules in Danish can vary a little depending on whether someone follows traditional comma or new comma conventions, but this punctuation is completely normal and helpful for learners.

Because the sentence refers to a specific novel, not just any novel.

• en roman = a novel
• romanen = the novel

The idea is that she is about to open a particular novel whose title she has just read. That is why the definite form makes sense.

Yes, it is perfectly natural in Danish to say åbne romanen if you mean opening the novel in order to start reading it.

Danish often allows the more specific noun:

• åbne bogen = open the book
• åbne romanen = open the novel

Both are possible, but romanen fits well because the sentence is specifically about a novel.

Yes. Danish normally repeats the subject in the new clause.

• Min søster er nysgerrig, så hun læser titlen igen

Even though hun clearly refers back to min søster, Danish still needs the pronoun in the second clause. You cannot usually omit it the way some languages can.

So repeating hun is required and natural.