A mi hermana le da igual que la reunión sea por la mañana o por la tarde.

Yes — they both refer to my sister, but this is a very normal Spanish pattern called indirect object doubling.

• A mi hermana names the person clearly.
• le is the indirect object pronoun that Spanish normally uses with verbs like dar.

So Spanish often says both:

• A mi hermana le da igual...

This is not redundant in the way it would be in English. It sounds natural and standard.

You can think of it like:

• As for my sister, it makes no difference to her...

The longer phrase A mi hermana can add clarity, contrast, or emphasis.

Dar igual is a very common expression meaning:

• to make no difference
• to not matter
• to be all the same to someone

So:

• Le da igual = It makes no difference to her / She doesn’t mind

A few examples:

• Me da igual. = I don’t mind.
• Nos da igual. = We don’t mind.
• ¿Te da igual? = Do you mind? / Is either option fine with you?

Even though the verb is dar (to give), you should learn dar igual as a set expression.

Because the thing that makes no difference is treated as one whole idea:

• que la reunión sea por la mañana o por la tarde

That entire clause is the subject of da igual, and it is understood as a single possibility/question, so da is singular.

In other words, the structure is not really counting morning and afternoon as two separate subjects. It is treating the whole choice as one matter.

Compare:

• Le da igual que venga hoy o mañana.
• Me da igual que sea rojo o azul.

In both cases, the whole whether X or Y idea is singular.

Here que introduces a subordinate clause:

• que la reunión sea por la mañana o por la tarde

It is similar to English that or sometimes whether, depending on how you translate it.

After expressions like me da igual, Spanish often uses:

• que + subjunctive

So:

• Me da igual que vengas hoy o mañana.
• Le da igual que la reunión sea por la mañana o por la tarde.

In this sentence, que links it doesn’t matter to her with the thing that does not matter.

Because after dar igual, Spanish commonly uses the subjunctive when talking about possibilities, options, or something not presented as an established fact.

Here, the meeting time is not being stated as a fact. It is an open option:

• it might be in the morning
• or it might be in the afternoon

That kind of uncertainty / non-factual situation often triggers the subjunctive.

So:

• que la reunión sea... = correct

This is very common after expressions of emotion, judgment, preference, doubt, or indifference, and dar igual fits that pattern.

In this sentence, sea is the expected choice.

• A mi hermana le da igual que la reunión sea... sounds natural and correct.

Using es here would generally sound wrong in standard grammar, because dar igual normally leads into a subjunctive clause with que.

So:

• le da igual que la reunión sea... ✅
• le da igual que la reunión es... ❌

A learner should strongly prefer the subjunctive after dar igual + que.

Because mi hermana is not the subject here. She is the person affected — the person to whom it makes no difference.

That is why Spanish uses:

• A mi hermana = to my sister
• le = to her

The real grammatical subject is the clause:

• que la reunión sea por la mañana o por la tarde

So the structure is roughly:

• To my sister, it makes no difference that/whether the meeting is in the morning or in the afternoon.

If you said Mi hermana le da igual..., it would suggest that my sister is the thing that is indifferent or irrelevant, which is not the meaning here.

In Spanish, por la mañana, por la tarde, and por la noche are the standard ways to say:

• in the morning
• in the afternoon
• at night / in the evening

So:

• La reunión es por la mañana.
• Trabajo por la tarde.

For a learner of Spanish from Spain, these are the forms you should definitely learn and use.

You may sometimes hear en la mañana in some parts of Latin America, but por la mañana is the normal and safest choice, especially for Spain.

Spanish uses the definite article much more often than English.

• la reunión = the meeting
• la mañana = the morning
• la tarde = the afternoon

In the expressions por la mañana and por la tarde, the article is simply part of the standard fixed expression.

So even though English often says just in the morning, Spanish says:

• por la mañana

You should learn these as full chunks:

• por la mañana
• por la tarde
• por la noche

Yes. Spanish word order is more flexible than English.

For example, these are possible:

• A mi hermana le da igual que la reunión sea por la mañana o por la tarde.
• Le da igual a mi hermana que la reunión sea por la mañana o por la tarde.

The original version is very natural because A mi hermana is placed first as the topic: as for my sister...

Putting A mi hermana first can help frame the sentence clearly, especially if you are contrasting her with someone else:

• A mi hermana le da igual, pero a mí no.

Yes, often you can, and the meaning is very similar.

• A mi hermana no le importa que la reunión sea por la mañana o por la tarde.

This means roughly:

• My sister doesn’t care whether the meeting is in the morning or in the afternoon.

But there is a slight nuance:

• le da igual often feels like either option is fine
• no le importa can sound a bit more like she doesn’t care

In many everyday situations, they are interchangeable.