A mi hermana le dan náuseas cuando viaja en autobús, y a veces tiene que parar para no vomitar.

Yes, they both point to the same person, but this is very normal in Spanish. It is called indirect object doubling.

• le is the indirect object pronoun and is usually the core grammatical marker.
• A mi hermana adds clarification or emphasis: it tells you exactly who le refers to.

So Spanish often says both: A mi hermana le dan náuseas...

In English, this feels redundant, but in Spanish it sounds natural.

Because Spanish often uses a different structure from English for physical or emotional reactions. Instead of making the person the grammatical subject, Spanish often makes the feeling or symptom the subject.

So:

• le = to her
• dan = give
• náuseas = nausea / feelings of sickness

Literally, it is something like nausea comes to her or nausea gives itself to her, but in natural English the idea is simply she feels sick / she gets nauseous.

This pattern is common in Spanish:

• Le da miedo = It scares her / She feels afraid
• Le da vergüenza = It embarrasses her
• Le dan náuseas = She feels nauseous

Because the verb agrees with náuseas, which is plural.

In this sentence, the grammatical subject is náuseas, not mi hermana.

So:

• la náusea da = singular
• las náuseas dan = plural

In everyday Spanish, náuseas is normally used in the plural, so le dan náuseas is the usual form.

Because this sentence describes a habitual, repeated situation: whenever she travels by bus, she feels sick.

With cuando, Spanish usually uses:

• indicative for things that happen regularly or are seen as real
• subjunctive for future or not-yet-real situations

So here:

• cuando viaja en autobús = when / whenever she travels by bus

Compare:

• Cuando viaja en autobús, le dan náuseas. = a usual fact
• Cuando viaje en autobús mañana, quizá se maree. = a future occasion

Because Spanish often leaves subject pronouns out when the meaning is already clear.

Here, once mi hermana has been mentioned, it is easy to understand that:

• viaja = she travels
• tiene = she has / she has to

Adding ella is possible, but it would usually add emphasis or contrast:

• Ella viaja en autobús, no en tren.

Without emphasis, Spanish normally omits it.

Because when Spanish talks about a means of transport in a general way, it often uses en + singular noun with no article.

So:

• en autobús = by bus
• en tren = by train
• en coche = by car
• en avión = by plane

If you say en el autobús, it usually means on the bus / inside the bus, focusing on location rather than the mode of transport.

So:

• Viaja en autobús = she travels by bus
• Está en el autobús = she is on the bus

In Spain, autobús is the standard, neutral word and is perfectly natural.

You may also hear:

• bus, which exists but is generally more informal
• autocar, especially for a coach or long-distance bus

So in a learner sentence, autobús is a very good and standard choice for Spain.

Tener que + infinitive is the normal everyday way to say to have to.

So:

• tiene que parar = she has to stop

It expresses necessity or obligation. In many contexts, it overlaps with English must, but the closest basic meaning is usually have to.

Compare:

• Tiene que parar = she has to stop
• Debe parar = she should stop / she ought to stop / she must stop, depending on context

For everyday necessity, tener que is extremely common.

Here parar means to stop or to interrupt the journey.

The sentence does not spell out the exact situation, so it could mean different things depending on context:

• if she is driving, she may need to pull over
• if she is a passenger, she may need the bus to stop or may need to get off briefly

The key idea is simply that she cannot continue normally for a moment because she feels so sick.

Because para + infinitive is a very common way to express purpose when the subject is the same in both parts of the sentence.

Here, the same person does both actions:

• she has to stop
• she wants to avoid vomiting

So:

• tiene que parar para no vomitar = she has to stop so as not to vomit / in order not to vomit

The no goes before the infinitive:

• para no vomitar

If the subject changed, Spanish would usually use para que + subjunctive:

• Pararon el autobús para que ella no vomitara.

Yes, absolutely. In Spain, marearse is very common for getting dizzy, queasy, or motion sick.

So:

• Se marea cuando viaja en autobús = she gets travel sick / she feels dizzy or queasy when she travels by bus

But there is a nuance:

• marearse focuses more on dizziness or motion sickness in general
• tener / darle náuseas focuses specifically on nausea

Since this sentence also mentions vomitar, náuseas fits very well because it highlights the nausea more directly.

Not strictly. In Spanish, a comma before y is often omitted:

• A mi hermana le dan náuseas cuando viaja en autobús y a veces tiene que parar...

However, some writers do use a comma before y when:

• the first part is fairly long
• there is already internal complexity
• they want to mark a clearer pause

So the comma here is not shocking, but many people would also write the sentence without it.