Mi sobrina se hizo una trenza muy bonita para la fiesta.

Because hacerse is often used when someone does something to themselves.

So mi sobrina se hizo una trenza means my niece made herself a braid / did her hair in a braid.

If you said mi sobrina hizo una trenza, it would more naturally sound like she made a braid in a more general sense, without clearly showing that it was on her own hair.

Here, se refers back to mi sobrina.

It is a common expression meaning to braid one’s hair or to put one’s hair in a braid.

Literally, it is something like to make oneself a braid, but in natural English that sounds awkward. In Spanish, though, this structure is very normal for hairstyles and personal grooming.

Similar patterns are:

• hacerse una coleta = to put one’s hair in a ponytail
• hacerse las uñas = to do one’s nails
• hacerse un tatuaje = to get a tattoo

Spanish usually needs an article here.

So una trenza means a braid. In this kind of sentence, Spanish normally does not drop the article the way English sometimes might.

Compare:

• Se hizo una trenza = She did her hair in a braid
• Llevaba una trenza = She was wearing a braid

Using una makes the hairstyle sound like a single, countable style element.

Hizo is the preterite form of hacer.

In this sentence, it shows a completed action in the past:

• Mi sobrina se hizo una trenza = My niece braided her hair / put her hair in a braid

The infinitive is hacer and the preterite forms are irregular:

• yo hice
• tú hiciste
• él/ella hizo
• nosotros hicimos
• vosotros hicisteis
• ellos hicieron

In Spanish, most descriptive adjectives normally come after the noun.

So:

• una trenza muy bonita = a very pretty braid

That is the neutral, standard order.

If an adjective comes before the noun, it often adds a more literary, emotional, or subjective tone. But here, trenza muy bonita is the most natural everyday order.

Muy means very and it modifies the adjective bonita.

So:

• bonita = pretty
• muy bonita = very pretty

It does not change for gender or number:

• muy bonito
• muy bonita
• muy bonitos
• muy bonitas

Only the adjective changes; muy stays the same.

Because bonita agrees with trenza, and trenza is a feminine singular noun.

So:

• una trenza bonita
• dos trenzas bonitas

The adjective must match the noun in gender and number.

Usually, yes: it suggests that she had her hair braided or arranged on herself.

In many contexts, a native speaker will understand that she did it to herself, especially in a simple sentence like this.

However, Spanish hacerse can sometimes also be used when someone has something done, depending on context. So if you want to make it absolutely clear that she personally braided her own hair, context helps.

Still, the most natural first interpretation here is that she braided her own hair.

It means for the party.

It gives the purpose or occasion:

• she made the braid for the party
• in other words, because she was going to the party, or as preparation for it

It does not necessarily mean she braided it at the party. If you wanted that idea, you would more likely use something like en la fiesta.

Compare:

• para la fiesta = for the party
• en la fiesta = at the party

Mi sobrina is the normal and most common way to say my niece.

Spanish usually uses the possessive adjective directly before the noun:

• mi sobrina
• mi hermano
• mi casa

You can say la sobrina mía, but that structure is much less neutral and usually sounds more emphatic, contrastive, or stylistically marked.

So in an ordinary sentence, Mi sobrina is exactly what you would expect.

Yes.

In British English, plait is very common. In American English, braid is more common.

So:

• una trenza = a braid / a plait

For a learner of Spanish from Spain, the Spanish word itself is standard and normal.

Yes, it sounds natural in Spanish from Spain.

Everything in it is standard:

• Mi sobrina = normal possessive structure
• se hizo una trenza = natural way to talk about doing one’s hair
• muy bonita = standard adjective placement
• para la fiesta = natural expression of purpose

A speaker from Spain would understand it immediately and would not find anything strange about it.

That word order is much less natural and can sound confusing.

In una trenza muy bonita, muy bonita clearly describes trenza.

If you move muy bonita to the end:

• Mi sobrina se hizo una trenza para la fiesta muy bonita

it may briefly sound as if muy bonita is describing la fiesta, or it may simply feel awkward.

So the best order is:

• Mi sobrina se hizo una trenza muy bonita para la fiesta

That keeps the description of the braid clear and natural.