A mi hermana le gusta poner cebollino en la tortilla.

Yes, they both point to the same person, but they do different jobs.

• a mi hermana names the person clearly: to my sister / as for my sister
• le is the indirect object pronoun that Spanish normally uses with gustar

With gustar, using the pronoun is standard:

• Le gusta... = She likes... / It pleases her...

Then a mi hermana is added to clarify or emphasize who le refers to.

So this structure is very normal:

• A Juan le gusta el café.
• A mí me gusta leer.
• A ellos les gusta viajar.

Spanish often uses both the pronoun and the full noun phrase together.

Because the verb agrees with what is doing the pleasing, not with the person who likes it.

In this sentence, what pleases your sister is the action poner cebollino en la tortilla.

That whole infinitive phrase behaves like a singular idea, so Spanish uses gusta:

• A mi hermana le gusta poner cebollino...

Compare:

• Le gusta cocinar. → She likes cooking.
• Le gustan las cebollas. → She likes onions.

So:

• gusta = used with a singular noun or an infinitive
• gustan = used with a plural noun

Because after gustar, Spanish often uses an infinitive to express an activity someone likes doing.

So:

• le gusta poner cebollino = she likes putting chives

This is very common:

• Me gusta nadar. = I like swimming.
• Les gusta salir por la noche. = They like going out at night.

In English, we often use -ing after like, but Spanish uses the infinitive:

• to put
• to read
• to cook

A useful way to think about it is:

[person affected] + indirect object pronoun + gustar + thing/activity liked

So here:

• A mi hermana = the person affected
• le = to her
• gusta = is pleasing
• poner cebollino en la tortilla = putting chives in the omelette

A more literal English gloss would be:

• To my sister, putting chives in the omelette is pleasing.

That sounds unnatural in English, but it helps explain why gustar works differently from to like.

Because with gustar, the person who likes something is introduced with a.

So Spanish says:

• A mi hermana le gusta... not
• Mi hermana le gusta... in this meaning

Without a, mi hermana would look more like the subject of the sentence, which would change the grammar completely.

This a is very common with gustar and similar verbs:

• A Pedro le encanta el fútbol.
• A nosotros nos interesa la historia.

Because here cebollino is being used as a general ingredient, not as a specific, clearly identified bunch of chives.

Spanish often omits the article with ingredients or materials in this kind of context:

• poner azúcar en el café
• añadir sal a la sopa
• usar aceite de oliva

So poner cebollino en la tortilla means to put chives in the omelette, in a general sense.

You could use an article in other contexts, but here no article sounds natural.

Because poner algo en algo means to put something in something.

So:

• poner cebollino en la tortilla = to put chives in the omelette

Use en when something goes inside or as part of something:

• poner sal en la sopa
• poner hielo en la bebida

If you said a la tortilla, that would suggest something more like to the omelette, which is not the usual wording here.

In Spanish from Spain, tortilla usually means omelette, and very often people think of tortilla de patatas.

So in this sentence, a learner of Spanish from Spain would normally understand:

• tortilla = omelette / Spanish tortilla

If you wanted to be extra specific, you might say:

• tortilla francesa = plain omelette
• tortilla de patatas = Spanish potato omelette

In many parts of Latin America, tortilla can mean something different, such as a flatbread.

Yes. Poner and echar can both work here.

• poner = to put
• echar = to add / put in

In everyday speech, especially in Spain, echar is very common for ingredients:

• echar sal
• echar azúcar
• echar cebollino

So both are natural, though echar may sound a bit more colloquial in cooking contexts.

Yes, Spanish word order is fairly flexible here.

These are all possible, depending on emphasis:

• A mi hermana le gusta poner cebollino en la tortilla.
• Le gusta poner cebollino en la tortilla a mi hermana.
• Poner cebollino en la tortilla le gusta a mi hermana.

But the first version is the most neutral and natural for learners.

Spanish often moves parts of the sentence around for focus, but the standard order is usually the safest choice.

Yes. For example:

• A ella le gusta poner cebollino en la tortilla. = She likes putting chives in the omelette.

You can also omit a ella if the context is clear:

• Le gusta poner cebollino en la tortilla.

But then le could mean to him, to her, or even to you (formal), so adding a ella makes it clear.

Because after gustar, Spanish directly uses the infinitive with no extra preposition.

So the pattern is:

• gustar + infinitive

Examples:

• Me gusta leer.
• Nos gusta cocinar.
• Le gusta poner cebollino...

English sometimes uses different patterns, but in Spanish you do not say:

• gusta de poner in normal modern usage here

Just use:

• le gusta poner...