Aunque sea un poco perezosa los domingos, mi hermana siempre cumple su palabra y termina sus tareas.

Because aunque can trigger either the subjunctive or the indicative, depending on the meaning.

Here, aunque sea un poco perezosa los domingos presents the laziness more as a concession: even if / although she may be a bit lazy on Sundays. The speaker is not focusing on that fact as the main point; they are setting it aside before saying something stronger about the sister.

So sea is the present subjunctive of ser.

• aunque sea... = although she may be... / even if she is...
• aunque es... = although she is... (more straightforwardly factual)

Both can be possible in some contexts, but sea gives a more concessive, slightly less direct tone.

Sea is the present subjunctive form of ser for yo, él/ella/usted.

In this sentence it refers to mi hermana, so it means:

• (ella) sea = she may be / she be

A quick mini-chart of the present subjunctive of ser:

• yo: sea
• tú: seas
• él/ella/usted: sea
• nosotros/as: seamos
• vosotros/as: seáis
• ellos/ellas/ustedes: sean

Because perezosa agrees with mi hermana, which is feminine singular.

In Spanish, adjectives usually match the noun they describe in gender and number:

• masculine singular: perezoso
• feminine singular: perezosa
• masculine plural: perezosos
• feminine plural: perezosas

Since hermana is feminine singular, the adjective must also be feminine singular: perezosa.

Los domingos means on Sundays or every Sunday.

Spanish often uses the definite article with days of the week to express a habitual action:

• los domingos = on Sundays / every Sunday
• los lunes = on Mondays

If you say just domingo or el domingo, that usually refers to a specific Sunday:

• el domingo = on Sunday / this Sunday / that Sunday, depending on context

So in your sentence, los domingos shows this is a general habit.

Because Spanish often drops subject pronouns when the verb ending already makes the subject clear.

In:

• mi hermana siempre cumple su palabra y termina sus tareas

the subject mi hermana has already been stated, so there is no need to add ella.

Adding ella would usually sound unnecessary unless you want emphasis or contrast:

• Mi hermana siempre cumple... = normal
• Mi hermana, ella siempre cumple... = more emphatic, and often less neutral

It is a very common expression meaning she keeps her word.

Literally, cumplir su palabra is something like to fulfill one’s word, but in natural English the best translation is:

• keep one’s word
• do what one promised

So:

• mi hermana siempre cumple su palabra = my sister always keeps her word

It is idiomatic and very natural Spanish.

Because the possessive su / sus agrees with the thing possessed, not with the owner.

Here:

• su palabra → singular, because palabra is singular
• sus tareas → plural, because tareas is plural

Both refer to mi hermana, but the form changes according to the noun that follows:

• su libro = her book
• sus libros = her books

This is different from English, where her does not change.

Because siempre usually goes before the conjugated verb when it modifies the whole action:

• siempre cumple
• siempre termina

That is the most neutral and common word order here.

You could also hear slightly different placements in other sentences, but mi hermana siempre cumple su palabra is the standard, natural order.

Because they belong to the main statement of the sentence, not to the concessive clause introduced by aunque.

The structure is:

• Aunque sea un poco perezosa los domingos,
  concessive clause → subjunctive
• mi hermana siempre cumple su palabra y termina sus tareas
  main clause → indicative

The speaker is making a real assertion about what the sister does: she does keep her word and does finish her tasks. That is why the indicative is used.

Yes, you could, and it would still be grammatical. The difference is mainly one of nuance.

• Aunque sea un poco perezosa...
  more concessive, slightly more like even if she may be a bit lazy...
• Aunque es un poco perezosa...
  more directly factual: although she is a bit lazy...

In many real-life contexts, both are possible. The subjunctive version often sounds a bit more elegant or more focused on brushing aside that detail before making the main point.

Un poco means a little or a bit.

So:

• un poco perezosa = a little lazy / a bit lazy

It softens the adjective, making the description less harsh. Compare:

• es perezosa = she is lazy
• es un poco perezosa = she is a bit lazy

This is very common in Spanish and sounds natural.

Because the sentence begins with a subordinate clause:

• Aunque sea un poco perezosa los domingos, ...

In Spanish, when a subordinate clause comes first, a comma is commonly used before the main clause. It helps the reader see the structure clearly.

So the comma separates:

• the concessive opening: Aunque sea un poco perezosa los domingos
• the main statement: mi hermana siempre cumple su palabra y termina sus tareas

Yes, it is natural and correct in Spanish from Spain.

It sounds like a normal written or careful spoken sentence. A speaker from Spain might also say similar versions such as:

• Aunque los domingos sea un poco perezosa, mi hermana siempre cumple su palabra y termina sus tareas.
• Aunque sea un poco vaga los domingos, mi hermana siempre cumple su palabra y termina sus tareas.

A small note: vaga is often more common in everyday speech than perezosa, depending on the speaker and context, but perezosa is perfectly natural and slightly more neutral or descriptive.

Yes. You could also say:

• Aunque los domingos sea un poco perezosa, mi hermana siempre cumple su palabra y termina sus tareas.

This is also very natural. Moving los domingos earlier can make the time expression connect more clearly with the whole first clause.

So both are good:

• Aunque sea un poco perezosa los domingos...
• Aunque los domingos sea un poco perezosa...

The meaning stays basically the same.