Mi hermana siempre lleva una horquilla extra por si el viento le mueve el pelo.

Llevar often means to carry / to have on you / to take with you.

So Mi hermana siempre lleva una horquilla extra means she habitually carries an extra hairpin with her.

• tener = to have, in a general sense
• llevar = to be carrying, wearing, or taking
• traer = to bring

Here, the idea is not just that she owns an extra hairpin, but that she keeps one with her, so lleva is the natural choice.

In Spanish, most descriptive adjectives normally come after the noun, especially when they simply classify or describe something.

So:

• una horquilla extra = an extra hairpin

That sounds natural. Putting extra before the noun would sound unusual here.

Also, extra is commonly used as an invariable adjective, so it does not change form:

• una horquilla extra
• dos horquillas extra

Por si means in case.

So:

• por si el viento le mueve el pelo = in case the wind moves her hair

It introduces a possible situation that she wants to be prepared for.

This is different from plain si, which usually just means if:

• Si hace viento, usa la horquilla = If it’s windy, she uses the hairpin.
• Lleva una horquilla por si hace viento = She carries a hairpin in case it gets windy.

So por si is about precaution/preparation.

Because por si is normally followed by the indicative, not the subjunctive.

So the sentence correctly says:

• por si el viento le mueve el pelo

This is a very common point for learners, because English in case can feel like it should trigger something more hypothetical, but in Spanish por si usually takes the indicative.

Examples:

• Lleva paraguas por si llueve.
• Te lo digo por si no lo sabes.

Not:

• por si llueva
• por si lo sepas

Le refers to mi hermana.

So literally, the structure is something like:

• the wind moves the hair to/from her
• more naturally: the wind moves her hair

Spanish often uses an indirect object pronoun with body parts:

• le duele la cabeza = her head hurts / she has a headache
• le lavó las manos = he washed her hands
• le mueve el pelo = it moves her hair

So le is not random; it marks the person affected.

Because with body parts, Spanish usually prefers:

• an indirect object pronoun for the person
• plus the definite article for the body part

So Spanish tends to say:

• le mueve el pelo
• me duele la cabeza
• se lavó las manos

rather than:

• mueve su pelo
• duele mi cabeza
• lavó sus manos

Using su pelo is grammatically possible in some contexts, but here le mueve el pelo is much more natural Spanish.

The subject is el viento.

So:

• el viento = the wind
• le = to her
• mueve = moves
• el pelo = the hair

Structure:

• el viento le mueve el pelo
• literally: the wind moves the hair on her
• natural English: the wind moves her hair

A learner might first think mi hermana is still the subject, but it is not. The verb mueve agrees with el viento.

Yes, that is possible, but it changes the nuance.

• el viento le mueve el pelo = the wind moves her hair
• se le mueve el pelo = her hair moves / gets moved on her

The version with el viento makes the wind the clear cause.
The version with se sounds less direct and focuses more on the hair moving.

Since the sentence explicitly mentions el viento, le mueve works well.

Yes, horquilla is a very normal word in Spain for a hairpin, especially the type used to hold hair in place.

Depending on the country, other words may appear, but for Spanish from Spain, horquilla is perfectly natural.

So if you are learning Peninsular Spanish, this is a useful word to know.

Siempre usually goes before the verb in neutral word order:

• Mi hermana siempre lleva una horquilla extra

That is the most standard placement for always here.

You may also hear different placements for emphasis, but this version is the normal one for everyday Spanish.

It describes a habit.

The present tense in Spanish is often used for habitual actions:

• Mi hermana siempre lleva... = My sister always carries...

So this does not mean she is necessarily carrying one at this exact second; it means this is something she regularly does.

The word siempre strongly supports that habitual meaning.