Hasta mi hermano, que siempre está tranquilo, se pone nervioso en la entrevista.

Here hasta means even.

So Hasta mi hermano... means Even my brother...

It adds emphasis: the speaker is saying that if even this person reacts that way, then the situation must really affect people.

Compare:

• Mi hermano se pone nervioso. = My brother gets nervous.
• Hasta mi hermano se pone nervioso. = Even my brother gets nervous.

Because this is extra, descriptive information about mi hermano.

The part que siempre está tranquilo means who is always calm, and it works like a non-defining relative clause in English. It is not used to identify which brother; it just adds extra information about him.

So:

• mi hermano = the main noun
• que siempre está tranquilo = extra comment about him

That is why it is set off with commas.

Spanish often uses estar for temporary states or how someone is being/feeling in a given sense, while ser is more about identity or more permanent characteristics.

In this sentence, está tranquilo suggests a state or usual condition of being calm. It sounds natural here because the contrast is with se pone nervioso — he becomes nervous.

So the sentence sets up this contrast:

• siempre está tranquilo = he is always calm
• se pone nervioso = he gets nervous

You could hear es tranquilo in other contexts to describe someone as a calm person by nature, but here está tranquilo fits very naturally with the idea of changing emotional state.

Ponerse nervioso means to become nervous or to get nervous.

It is a very common way in Spanish to talk about a change of state:

• ponerse triste = to become sad
• ponerse contento = to become happy
• ponerse rojo = to turn red
• ponerse nervioso = to get nervous

So:

• está tranquilo = he is calm
• se pone nervioso = he becomes nervous

This contrast is an important part of the sentence.

Because this is the pronominal verb ponerse, which often means to become when followed by an adjective.

Without se, poner usually means to put:

• Pongo el libro en la mesa. = I put the book on the table.

But:

• Me pongo nervioso. = I get nervous.
• Se pone nervioso. = He gets nervous.

So in this sentence, se pone nervioso is not he puts himself nervous in a literal English sense. It is just the normal Spanish structure for he gets nervous.

Because mi hermano is masculine singular, so the adjective must agree with it.

Agreement here:

• hermano → masculine singular
• nervioso → masculine singular
• tranquilo → masculine singular

If it were about a sister, it would change:

• Hasta mi hermana, que siempre está tranquila, se pone nerviosa en la entrevista.

Here en la entrevista is best understood as in the interview or during the interview, depending on how naturally you would say it in English.

Spanish often uses en where English might prefer in, at, or during, depending on context.

So:

• se pone nervioso en la entrevista = he gets nervous in/during the interview

In natural English, during the interview is often the clearest translation, but the Spanish structure with en is completely normal.

Spanish word order is fairly flexible, and here the sentence begins with Hasta mi hermano to emphasise that person.

The structure is:

• Hasta mi hermano = Even my brother
• que siempre está tranquilo = who is always calm
• se pone nervioso en la entrevista = gets nervous in the interview

Putting Hasta mi hermano first gives the sentence a strong emphasis from the start.

A more neutral structure without that emphasis would lose some of the effect:

• Mi hermano también se pone nervioso... = My brother also gets nervous...
• But Hasta mi hermano... specifically highlights surprise.

Not with the same meaning.

• también = also
• hasta = even

So:

• Mi hermano también se pone nervioso = My brother also gets nervous.
• Hasta mi hermano se pone nervioso = Even my brother gets nervous.

The second one sounds more surprising or emphatic. It suggests that the brother is the last person you would expect to get nervous.

Using la entrevista suggests a specific interview that both speaker and listener can identify from context.

So it usually means:

• the interview = a particular interview already known in the situation

If you said una entrevista, it would sound more general or less specific:

• se pone nervioso en una entrevista = he gets nervous in an interview / during interviews of that kind

In your sentence, la entrevista most naturally refers to one specific interview.

No. It refers to mi hermano.

Why?

• It comes right after mi hermano
• The adjective tranquilo is masculine singular, matching hermano
• Semantically, it makes sense that a person is tranquilo, not an interview

So the structure is clearly:

• mi hermano
  • que siempre está tranquilo

That means:

• my brother, who is always calm

Yes, because ponerse nervioso highlights the change into that state.

Compare:

• está nervioso = he is nervous
• se pone nervioso = he gets nervous / becomes nervous

In this sentence, the contrast is:

• normally calm
• but then he becomes nervous in the interview

So se pone nervioso is the better choice because it shows the transition from calmness to nervousness.