Mi hermana es muy puntual, aunque hoy ha llegado tarde porque tenía otro compromiso.

Because puntual here describes a general characteristic of your sister, not a temporary state.

• ser + adjective = what someone is like in general
• estar + adjective = a temporary condition or state

So:

• Mi hermana es muy puntual = My sister is a very punctual person / she is generally punctual.

Using está puntual would sound unusual in this context. If you wanted to talk about her being on time for a specific occasion, Spanish would more naturally say something like ha llegado puntual or está aquí a tiempo, depending on the situation.

Because muy is used to modify adjectives and adverbs, while mucho is used with nouns or as a pronoun/adverb in other contexts.

Here, puntual is an adjective, so you need:

• muy puntual = very punctual

Compare:

• muy puntual = very punctual
• mucho trabajo = a lot of work

So mucho puntual is incorrect.

Some Spanish adjectives have the same form for masculine and feminine.

So:

• un chico puntual
• una chica puntual
• mi hermano es puntual
• mi hermana es puntual

The adjective only changes in number here:

• puntual = singular
• puntuales = plural

So mi hermana es muy puntual is already correct.

Spanish often omits subject pronouns because the verb already shows who the subject is.

In this sentence:

• es already tells you it is she/he/it
• ha llegado also fits she
• the context makes it clear that the subject is mi hermana

So Spanish normally says:

• Mi hermana es muy puntual...

not

• Mi hermana ella es muy puntual...

Adding ella would usually only happen for emphasis, contrast, or clarification.

Aunque means although / even though.

It introduces a contrast:

• Mi hermana es muy puntual
  aunque
• hoy ha llegado tarde

So the idea is: she is usually punctual, although today she arrived late.

It takes the indicative here because the speaker is talking about a real, known fact:

• aunque hoy ha llegado tarde

If it were something uncertain or hypothetical, Spanish could use the subjunctive:

• Aunque llegue tarde, no pasa nada.
  = Even if she arrives late, it’s not a problem.

This is a very common Spain Spanish pattern.

In much of Spain, the present perfect is often used for actions in a time period that is still connected to the present, especially with words like:

• hoy = today
• esta semana = this week
• este mes = this month

So in Spain:

• Hoy ha llegado tarde sounds very natural.

In many parts of Latin America, people would more often say:

• Hoy llegó tarde

Both can be correct depending on regional usage, but ha llegado is especially typical of Spain.

It is the present perfect:

• ha = present tense of haber
• llegado = past participle of llegar

So:

• ha llegado = has arrived / arrived

The full pattern is:

• he llegado
• has llegado
• ha llegado
• hemos llegado
• habéis llegado
• han llegado

Remember that with compound tenses in Spanish, the auxiliary is haber, not ser or estar.

Tenía is the imperfect, and here it gives background information or explains the situation.

The sentence structure is:

• She arrived late
• because she had another commitment

Spanish often uses the imperfect for the circumstance behind another action:

• ha llegado tarde porque tenía otro compromiso

This sounds like background: there was an existing obligation that explains the lateness.

If you said tuvo otro compromiso, it would sound more like a completed event or a specific occurrence, which is less natural here.

So:

• tenía = she had / she was dealing with / she already had

is the normal choice.

Here, compromiso means something like:

• commitment
• engagement
• appointment
• obligation
• prior arrangement

So tenía otro compromiso means she had something else she had to attend to.

Be careful: compromiso does not always mean a romantic engagement. Its meaning depends on context.

Examples:

• Tengo un compromiso esta tarde. = I have an appointment/commitment this afternoon.
• Es un compromiso importante. = It’s an important obligation/engagement.

Because in Spanish, otro normally goes directly before the noun without un.

So you say:

• otro compromiso = another commitment

not

• un otro compromiso

This is the normal rule with otro/a/os/as:

• otra idea
• otros planes
• otras personas

English uses another, and Spanish usually expresses that with just otro + noun.

Because tarde can have different meanings depending on context.

Here it appears in the expression:

• llegar tarde = to arrive late

So in this sentence, tarde is an adverb, meaning late.

But la tarde as a noun means the afternoon.

Compare:

• Ha llegado tarde. = She arrived late.
• Por la tarde. = In the afternoon.

Same word, different function.

Yes, it is a very common expression.

• llegar tarde = to arrive late
• llegar pronto = to arrive early/soon
• llegar a tiempo = to arrive on time

Examples:

• Siempre llego tarde. = I always arrive late.
• Hoy hemos llegado a tiempo. = Today we arrived on time.

So in your sentence, ha llegado tarde is a very natural and everyday combination.

Because aunque hoy ha llegado tarde porque tenía otro compromiso is an added clause that contrasts with the first idea.

The comma helps separate:

• Mi hermana es muy puntual from
• aunque hoy ha llegado tarde porque tenía otro compromiso

This kind of comma is very common when a sentence introduces a contrasting idea after the main statement.

You may sometimes see variation in punctuation depending on style, but in a sentence like this, the comma is very natural and helpful.

Yes, Spanish allows some flexibility, though the original order is the most neutral.

Original:

• Mi hermana es muy puntual, aunque hoy ha llegado tarde porque tenía otro compromiso.

You could also say:

• Aunque hoy ha llegado tarde porque tenía otro compromiso, mi hermana es muy puntual.

This version puts the contrast first, but the meaning stays basically the same.

Spanish word order is often changed for focus, style, or emphasis, but the original sentence sounds very natural and straightforward.

Not necessarily wrong, but they would sound different.

• porque tenía otro compromiso = the most natural here; it gives background
• porque tuvo otro compromiso = sounds more like a specific completed event
• porque ha tenido otro compromiso = possible in some contexts, but less natural in this sentence

Since the other commitment is the circumstance explaining why she arrived late, Spanish usually prefers the imperfect:

• tenía

That tense paints the situation as an ongoing or pre-existing fact in the background.

The sentence is already very natural, but a learner might hear slight alternatives such as:

• Mi hermana suele ser muy puntual, pero hoy ha llegado tarde porque tenía otro compromiso.
• Mi hermana es muy puntual, pero hoy llegó tarde porque tenía otra cosa.

These are just variations:

• suele ser adds the idea of usually
• pero is a more direct but
• otra cosa is less specific than otro compromiso

Your original sentence is perfectly natural and a good model of standard Spanish from Spain.