Mi hermana se fue tranquilizando cuando vio que la portera seguía muy segura.

Se fue tranquilizando shows a gradual change: she was calming down little by little.

By contrast, se tranquilizó presents it more as a single completed change: she calmed down.

So:

• se fue tranquilizando = the process matters
• se tranquilizó = the result matters more

This use of ir + gerund is very common when Spanish wants to show something developing progressively over time.

Here, se is part of the pronominal verb tranquilizarse, which means to calm down or to become calm.

Compare:

• tranquilizar a alguien = to calm someone down
• tranquilizarse = to calm down oneself / to become calmer

So:

• La madre tranquilizó a la niña. = The mother calmed the girl down.
• La niña se tranquilizó. = The girl calmed down.

In your sentence, mi hermana se fue tranquilizando means your sister herself gradually became calmer.

Ir + gerund often expresses a process that unfolds step by step or progressively.

In this sentence:

• fue = from ir
• tranquilizando = gerund of tranquilizar

Together, se fue tranquilizando means something like:

• she gradually calmed down
• she was getting calmer and calmer

This structure is especially useful when the speaker wants to show that the change was not immediate.

Other examples:

• La situación fue mejorando. = The situation gradually improved.
• Me fui acostumbrando. = I gradually got used to it.

Vio is the preterite of ver, and it is used because the sentence refers to a specific moment when she saw something.

So cuando vio means when she saw at that particular point in time.

If you said cuando veía, it would usually suggest a more repeated, habitual, or ongoing meaning, which does not fit as naturally here.

Compare:

• cuando vio que... = when she saw that...
• cuando veía que... = when she would see / while she was seeing that...

In this sentence, the seeing is the triggering event, so vio is the natural choice.

Seguía is the imperfect of seguir, and here it describes an ongoing background state.

The idea is that at the moment your sister saw her, the porter was still looking very sure/confident. That sense of ongoing continuation is exactly what the imperfect does well.

So:

• seguía muy segura = she still seemed/remained very sure

If you used siguió, it would sound more like a completed event in the foreground, which is less natural in this context.

A useful way to see the contrast:

• vio = the main event
• seguía = the continuing situation she noticed

Seguir + adjective means to continue being or to remain in a certain state.

So:

• seguía muy segura = she remained very sure / she was still very confident

This is a very common pattern in Spanish:

• sigue abierto = it is still open
• seguían cansados = they were still tired
• sigo enfermo = I am still ill

Here it tells you that the porter’s confidence did not change; it continued.

In Spain, la portera often means the female concierge / doorkeeper / caretaker of a building.

That is probably the intended meaning here, especially in a Spanish setting.

Although portera can also mean female goalkeeper, context matters. In this sentence, unless the wider context is about sports, a learner should usually understand la portera as the building porter/caretaker.

Here, segura means sure, certain, or confident, not safe.

With people, estar seguro/a often means:

• to be sure
• to be confident
• to feel certain

So la portera seguía muy segura means the porter still seemed very certain/confident.

If Spanish wants to emphasize safe in a physical sense, context usually makes that clear:

• Estoy segura aquí. could mean I am safe here
• Estoy segura de que... clearly means I am sure that...

In your sentence, the idea is confidence or certainty.

Because muy is used to modify adjectives and adverbs, while mucho is generally used with nouns or as a pronoun/adverb meaning a lot / much.

Here, segura is an adjective, so Spanish needs muy:

• muy segura = very sure/confident

Compare:

• muy cansada = very tired
• muy bien = very well
• mucho tiempo = a lot of time
• mucha gente = many people

So mucho segura is not correct.

Because que introduces the subordinate clause that explains what she saw.

Structure:

• vio = she saw
• que la portera seguía muy segura = that the porter was still very sure

So ver que... means to see that..., in the sense of to realize / notice that...

Examples:

• Vi que no venía nadie. = I saw that nobody was coming.
• Vio que todo estaba bien. = He/She saw that everything was fine.

This is a very common Spanish structure.

Yes. Spanish is more flexible with word order than English.

For example, you could also say:

• Cuando vio que la portera seguía muy segura, mi hermana se fue tranquilizando.

This keeps the same basic meaning. The difference is mainly one of focus or style:

• Mi hermana se fue tranquilizando cuando... starts with your sister and the change in her state
• Cuando vio que..., mi hermana... starts with the triggering moment

Both are natural.

Because Spanish often combines past tenses to show the relationship between:

• a main event
• an ongoing background situation

Here:

• vio = preterite, a completed event
• seguía = imperfect, an ongoing state
• se fue tranquilizando = a progressive change unfolding after that moment

So the sentence paints a timeline like this:

1. the porter was still very confident
2. your sister saw that
3. as a result, your sister gradually calmed down

This is one of the most important patterns in Spanish narration: preterite for events, imperfect for background/continuing states.