Hvor þeirra kemur með okkur á fundinn, hún eða systir hennar?

Hvor is used when the choice is between two specific options. That fits this sentence because the speaker is asking which of two people it is: hún eða systir hennar.

By contrast, hver is the more general word for who / which / each, and it is not limited to just two choices.

So here:

• hvor = which of the two
• hver = who / which more generally

Þeirra is the genitive plural form of the 3rd-person pronoun: their / of them.

In this sentence, hvor þeirra literally means which of them. Icelandic often uses this kind of partitive genitive after words like hvor to mean which one out of a group.

So:

• þau / þær / þeir = they
• þeirra = of them

Even though the two people here are women, þeirra is still the correct genitive plural form used in this structure.

Because the subject is understood as one person: which one of them.

Even though þeirra refers to more than one person, the actual question is about a single person coming. So Icelandic uses the 3rd person singular verb:

• kemur = comes / is coming

If the subject were truly plural, then you would use a plural verb form.

This follows a very common Icelandic pattern: in a main clause question, the question word comes first, and the finite verb comes next.

So the structure is:

• Hvor þeirra = question phrase
• kemur = finite verb
• then the rest of the sentence

This is part of Icelandic’s verb-second tendency in main clauses.

Because með takes the dative case when it means with in the sense of accompaniment.

The pronoun við means we / us, but its form changes by case:

• nominative: við
• dative: okkur

So:

• með okkur = with us

This is just normal case government after the preposition með.

There are two things happening here:

1. fundinn is the accusative singular definite form of fundur

   • fundur = meeting
   • fundinn = the meeting

2. The preposition á takes the accusative when it shows movement toward a place/event, but the dative when it shows location.

So:

• á fundinn = to the meeting / going to the meeting
• á fundinum = at the meeting

That is why á fundinn is used here: the person is coming along to the meeting.

Because hún is standing as one of the possible subjects of kemur.

The sentence is basically asking:

• Which one is coming — she or her sister?

Since these are subject alternatives, Icelandic uses the nominative:

• hún = she
• not hana = her

English often blurs this in speech, but Icelandic keeps the case distinction clearly.

Systir hennar means her sister.

In Icelandic, possessive forms like hennar often come after the noun:

• systir hennar = her sister
• literally: sister her

This is a very normal Icelandic word order.

Also:

• hennar is the genitive form meaning her
• it shows who the sister belongs to

Not always. It is there to spell out the two options clearly.

You could say just:

• Hvor þeirra kemur með okkur á fundinn?

if the two people are already obvious from context.

Adding:

• hún eða systir hennar

makes the contrast explicit and removes ambiguity. It is similar to English saying:

• Which of them is coming with us to the meeting — she or her sister?

Yes. It sounds like a normal, natural way to ask which of two women is coming along to a meeting.

The sentence is especially clear because it:

• uses hvor for a two-way choice
• uses hvor þeirra for which of them
• then names the two alternatives explicitly at the end

So structurally, it is a very good example of a real Icelandic question.