Mi hermana se negó a mentir en la entrevista.

Because the verb here is negarse, which means to refuse.

The se is part of the verb, not a separate optional word. So:

• negar = to deny
• negarse = to refuse

That difference is very important:

• Mi hermana se negó a mentir = My sister refused to lie
• Mi hermana negó... = My sister denied...

So in this sentence, se negó is the correct form because the meaning is refused.

Se negó is the third-person singular preterite form of negarse.

It matches mi hermana, because mi hermana = she.

So:

• yo me negué = I refused
• tú te negaste = you refused
• él/ella se negó = he/she refused
• nosotros nos negamos = we refused
• vosotros os negasteis = you all refused
• ellos/ellas se negaron = they refused

In your sentence, mi hermana is singular, so se negó is the right form.

Because negarse is normally followed by a + infinitive.

The pattern is:

negarse a + infinitive

So:

• se negó a mentir = refused to lie
• me niego a esperar = I refuse to wait
• se negaron a entrar = they refused to go in

That a is required with this structure. It is not the same as the English to exactly, but in this pattern it helps express refuse to do something.

Because after a in this structure, Spanish uses the infinitive.

So:

• se negó a mentir = refused to lie

Not:

• se negó a mintió
• se negó a mentía
• se negó a mentido

After negarse a, you use the base dictionary form of the verb, which is the infinitive: mentir.

It means to lie in the sense of not telling the truth.

Spanish separates those ideas:

• mentir = to lie, to tell a falsehood
• tumbarse / acostarse = to lie down

So mentir can only mean to tell a lie here.

En la entrevista means in the interview or during the interview.

Here, en gives the setting or context where the action would happen. So the sentence means she refused to lie in that situation.

You could also hear:

• durante la entrevista = during the interview

That is a bit more explicit, but en la entrevista sounds very natural.

In Spanish, the definite article is often used where English might leave it out.

So Spanish naturally says:

• en la entrevista
• en la reunión
• en la clase

Even if English might say something like in the interview or sometimes just at interview in a more general sense, Spanish usually prefers the article here.

Negó is in the preterite.

The preterite is used for a completed action in the past. Here, it presents her refusal as a single finished event:

• Mi hermana se negó a mentir en la entrevista = she refused at that specific moment/situation

If you used the imperfect instead:

• Mi hermana se negaba a mentir...

that would suggest something more ongoing, repeated, or descriptive, such as she would refuse to lie or she was refusing to lie depending on context.

So the preterite fits a single completed incident very well.

The accent mark shows the stress and is part of the correct spelling.

In the preterite, many -ar verbs in the él/ella/usted form end in -ó:

• habló
• miró
• negó

So:

• se negó = he/she refused

The accent tells you the stress falls on the last syllable: ne-GÓ.

No. That would not be correct for this meaning.

If you want to say refused to lie, you need:

• Mi hermana se negó a mentir

Without se, negar usually means to deny something, not to refuse.

For example:

• Negó la acusación = She denied the accusation
• Negó haber mentido = She denied having lied

So this is a very important contrast:

• negar algo = to deny something
• negarse a hacer algo = to refuse to do something

Yes. Spanish often omits subject pronouns and even full subjects when the meaning is clear from context.

So if everyone already knows you are talking about your sister, you could simply say:

• Se negó a mentir en la entrevista.

Spanish does this a lot because the verb form already tells you the person and number. Here, negó tells you it is he/she/you formal singular.

The full subject Mi hermana is included for clarity, contrast, or emphasis.

The given word order is the most neutral and natural:

• Mi hermana se negó a mentir en la entrevista.

But Spanish does allow some flexibility.

For example:

• En la entrevista, mi hermana se negó a mentir.

This puts more focus on the interview as the setting.

You would not normally move things around randomly, but Spanish is a bit more flexible than English because endings and structure make relationships clearer.

Yes. It is fully natural in Spain Spanish.

Nothing in the sentence is region-specific or unusual. A speaker in Spain would understand and use this structure naturally:

• negarse a + infinitive
• mentir
• en la entrevista

So this is standard, everyday Spanish that works well in Spain.