Aunque a veces sea perezosa los domingos, mi hermana siempre termina sus tareas.

Because aunque can be followed by either the subjunctive or the indicative, depending on how the speaker sees the information.

Here, sea is the present subjunctive of ser. It suggests something like even if / although she may be lazy on Sundays sometimes. The speaker is presenting that idea as a concession, not as the main point.

• Aunque a veces sea perezosa... = concessive, slightly less direct, more general
• Aunque a veces es perezosa... = more factual, as if the speaker is simply stating a known fact

In this sentence, the important idea is that she still always finishes her tasks.

Aunque means although, even though, or sometimes even if, depending on context.

In this sentence, it introduces a contrast:

• Aunque a veces sea perezosa los domingos = Although she is sometimes lazy on Sundays
• mi hermana siempre termina sus tareas = my sister always finishes her tasks

So the overall idea is: there is a possible obstacle or contrasting fact, but the main result still happens.

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

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

• hermano perezoso = lazy brother
• hermana perezosa = lazy sister

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

Since the sentence uses ser — sea perezosa — it sounds more like a characteristic or tendency, not just a momentary condition.

So this suggests something like:

• she can be lazy on Sundays
• she has that tendency sometimes

If you wanted to emphasize a temporary state, Spanish often prefers estar:

• Aunque a veces esté perezosa los domingos...

That would sound more like although she feels lazy on Sundays sometimes.

So the original sentence is understandable, but learners should notice that ser perezosa and estar perezosa are not exactly the same.

In Spanish, days of the week often take the definite article to mean on Sundays, on Mondays, etc.

So:

• los domingos = on Sundays / every Sunday
• el domingo = on Sunday / this Sunday, depending on context

You normally do not say en los domingos for this meaning.

This is a very common Spanish pattern:

• Trabajo los lunes. = I work on Mondays.
• Descansamos los fines de semana. = We rest on weekends.

A veces means sometimes.

Here it modifies sea perezosa, so it tells us that this does not happen all the time:

• Aunque a veces sea perezosa los domingos... = Although she is sometimes lazy on Sundays...

Its position is fairly flexible, but some positions sound more natural than others. For example:

• Aunque a veces sea perezosa los domingos...
• Aunque sea perezosa a veces los domingos...
• Aunque los domingos a veces sea perezosa...

The original placement is normal and easy to understand.

Because the main clause is stating a straightforward fact or habitual action:

• mi hermana siempre termina sus tareas

This means my sister always finishes her tasks.

The subjunctive appears in the aunque clause because that clause expresses concession. But the main statement is presented as real and certain, so Spanish uses the indicative:

• termina = present indicative of terminar

So the sentence mixes:

• subjunctive in the subordinate clause
• indicative in the main clause

That combination is very common.

Sus tareas means her tasks.

Spanish often uses possessive adjectives where English would too. Since mi hermana is the person being talked about, sus clearly refers to her.

• sus tareas = her tasks

You could sometimes hear las tareas if the context already makes it obvious whose tasks they are, but sus tareas is more explicit and natural here.

Also remember that sus can mean:

• his
• her
• your (formal)
• their

The context tells you which one is intended.

No. Siempre termina sus tareas means she always ends up finishing her tasks or she always finishes her tasks, but it does not say when.

It only tells us that the tasks get completed in the end.

If you wanted to say she finishes them quickly, you would need extra words, for example:

• siempre termina sus tareas rápido
• siempre termina enseguida sus tareas

So in the original sentence, the focus is on completion, not speed.

It sounds like a general habit or repeated pattern, not one specific Sunday.

There are several clues:

• a veces = sometimes
• los domingos = on Sundays / every Sunday
• siempre termina = she always finishes

All of that points to a recurring situation: sometimes she is lazy on Sundays, but she still always gets her tasks done.

If it were about one specific Sunday, Spanish would probably be more specific, for example:

• Aunque hoy esté perezosa, mi hermana terminará sus tareas.

Yes, the comma is appropriate here.

The sentence begins with a subordinate concessive clause:

• Aunque a veces sea perezosa los domingos,

Then it moves to the main clause:

• mi hermana siempre termina sus tareas.

In Spanish, when a subordinate clause comes first, it is normal to separate it from the main clause with a comma. So the punctuation in the original sentence is correct and helpful.