Hun er lige så grundig som sin mor og endnu mere nøjagtig end sin bror.

Because Danish uses different comparison words for different kinds of comparison:

• lige så + adjective + som = as ... as
• mere + adjective + end = more ... than

So in this sentence:

• lige så grundig som sin mor shows equality
• mere nøjagtig end sin bror shows difference

A good way to remember it is:

• som goes with equal comparison
• end goes with comparative comparison

Lige strengthens the idea of equality. It makes the comparison feel like just as or exactly as.

So:

• så grundig som = as thorough as
• lige så grundig som = just as / equally thorough as

In modern Danish, lige så ... som is a very common and natural pattern.

Because sin is the reflexive possessive. It is used when the possessor is the same as the subject of the clause.

Here, the subject is Hun, so:

• sin mor = her own mother
• sin bror = her own brother

If you said hendes mor or hendes bror, it would usually mean the mother or brother of some other woman, not the subject hun.

So the contrast is:

• Hun elsker sin mor. = She loves her own mother.
• Hun elsker hendes mor. = She loves that other woman’s mother.

Not exactly. Sin refers to the subject of its own clause.

In this sentence, there is only one main clause, so both sin words refer back to hun.

That matters in longer sentences. For example:

• Hun siger, at hendes bror elsker sin hund.

Here:

• hendes bror refers to her brother
• sin hund refers to the brother’s own dog, because bror is the subject of the subordinate clause at ... elsker sin hund

So the key rule is: sin points back to the subject of the clause it is inside.

They are predicate adjectives, because they come after er.

In Danish, predicate adjectives agree with the subject. Since hun is singular and not neuter, the adjective stays in its basic form:

• Hun er grundig.
• Hun er nøjagtig.

Compare:

• Et arbejde er grundigt. — neuter singular, so -t
• De er grundige. — plural, so -e

So in your sentence, the basic forms grundig and nøjagtig are exactly what you expect after hun er.

Yes, nøjagtigere is grammatically possible.

Danish can often form comparatives in two ways:

• with -ere
• with mere

For longer adjectives, speakers often prefer mere + adjective, because it can sound smoother and less heavy.

So:

• mere nøjagtig
• nøjagtigere

are both possible, but mere nøjagtig is very natural here.

Here, endnu is an intensifier. It means something like even or still in English.

So:

• mere nøjagtig = more accurate / more precise
• endnu mere nøjagtig = even more accurate / even more precise

This is not the same as endnu meaning yet in other contexts.

For example:

• Jeg er ikke færdig endnu. = I’m not finished yet.
• Hun er endnu mere nøjagtig. = She is even more precise.

Yes, you can think of it that way.

The full comparisons could be expanded to:

• Hun er lige så grundig som sin mor er
• Hun er endnu mere nøjagtig end sin bror er

But Danish often leaves out the repeated verb when it is obvious from the context. That omission is completely normal.

So the sentence is shorter, but the meaning is still clear.

Because the possessive already determines the noun.

In Danish, you normally do not use an article together with a possessive in this kind of structure:

• min bror
• hans mor
• sin søster

not:

• den min bror
• en sin mor

So sin mor and sin bror are the normal forms.

This is especially common and straightforward with family words.

They are related, but not identical.

• grundig = thorough, careful in covering everything
• nøjagtig = accurate, precise, careful about details and correctness

So someone can be:

• grundig because they work systematically and don’t miss steps
• nøjagtig because they are exact and precise

In this sentence, the speaker is saying that she is:

• as thorough as her mother
• even more precise than her brother

That pairing sounds very natural, because the two adjectives describe slightly different strengths.