Om hun så pludrer eller græder, holder min søster sig rolig og prøver igen at trøste hende.

Here om introduces a subordinate clause meaning whether.

So Om hun så pludrer eller græder means something like:

• Whether she babbles or cries
• No matter whether she babbles or cries

In Danish, om can introduce this kind of whether-clause, not just a question.

In this sentence, så does not mean so or then in the usual sense.

It is part of the pattern:

• om ... så ... eller ...

This gives the meaning whether ... or ..., often with a nuance like no matter whether.

So you should understand the whole expression together, rather than translating så by itself.

This is because Danish main clauses usually follow the V2 rule: the finite verb comes in the second position.

The first position here is already taken by the whole opening clause:

• Om hun så pludrer eller græder

So the main clause must begin with the finite verb:

• holder min søster sig rolig

That is why the subject min søster comes after holder.

Pludrer is the present tense of pludre.

It usually means:

• babble
• coo
• chatter

It is especially common for babies or small children making happy little sounds.

So hun pludrer means she is babbling/cooing or she babbles.

Because holde sig rolig means keep calm or stay calm, not just be calm.

Compare:

• Min søster er rolig = my sister is calm
• Min søster holder sig rolig = my sister keeps/stays calm

The second one suggests an active effort or continued self-control.

The word sig is reflexive, meaning the action refers back to the subject. So your sister is keeping herself calm.

Sig is the reflexive pronoun used when the object refers back to the subject.

Here:

• subject: min søster
• reflexive pronoun: sig

So holder sig rolig literally means something like keeps herself calm.

In Danish, reflexive pronouns are very common in expressions like:

• sætte sig
• føle sig
• holde sig rolig

The adjective agrees with the subject.

Here the subject is min søster, which is singular and common gender, so the form is:

• rolig

Compare:

• Min søster er rolig
• Barnet er roligt
• De er rolige

Even though sig appears in the sentence, the adjective matches the subject min søster, not the pronoun.

Because Danish normally uses prøve at + infinitive when it means try to do something.

So:

• prøver at trøste = tries to comfort

This is a very common pattern:

• Jeg prøver at forstå
• Hun prøver at sove
• Vi prøver at hjælpe

Igen means again.

Here:

• prøver igen at trøste hende = tries again to comfort her

It naturally comes after the finite verb prøver and before the infinitive phrase at trøste hende.

So the idea is that your sister makes another attempt.

Hende is the object form of hun, meaning her.

It is used because she is the object of trøste:

• trøste hende = comfort her

So:

• hun = she
• hende = her

This tells you that the person being comforted is female, or at least referred to with the feminine personal pronoun in the context.

Yes. Græder is the present tense of græde, meaning cry.

So:

• hun græder = she cries / she is crying

In this sentence, both pludrer and græder are in the present tense, giving two possible things she might be doing.

Because the opening part is a subordinate clause:

• Om hun så pludrer eller græder

In Danish, subordinate clauses are separated from the main clause with a comma. After that comes the main clause:

• holder min søster sig rolig og prøver igen at trøste hende

So the comma helps show the structure of the sentence.