Meine Schwester kocht Haferbrei, wenn sie früh zur Arbeit muss.

Meine Schwester is the subject of the main clause, so it is in the nominative case.

• meine Schwester = nominative, feminine singular
• meiner Schwester would be dative or genitive, depending on context

Here, the sister is the person doing the cooking, so nominative is required.

In a normal German main clause, the finite verb must be in second position. This is the famous verb-second rule.

So in:

Meine Schwester kocht Haferbrei

• position 1: Meine Schwester
• position 2: kocht

Everything else comes after that.

German often leaves out the article with mass nouns or food items when speaking generally.

So Haferbrei here means oatmeal or porridge in a general sense, not a specific previously mentioned bowl of it.

Compare:

• Meine Schwester kocht Haferbrei = she cooks oatmeal
• Meine Schwester kocht den Haferbrei = she cooks the oatmeal, meaning a specific one

Because wenn sie früh zur Arbeit muss is a subordinate clause, and German normally separates subordinate clauses from main clauses with a comma.

So the comma is not optional here. It is part of standard German punctuation.

Because wenn introduces a subordinate clause, and in German subordinate clauses usually send the finite verb to the end.

So:

• main clause: Meine Schwester kocht Haferbrei
• subordinate clause: wenn sie früh zur Arbeit muss

That is why muss appears at the end.

Wenn is used for repeated situations, general situations, or conditions.

In this sentence, the meaning is something like whenever she has to go to work early. That is a repeated or general situation, so wenn is correct.

Compare:

• wenn = when, whenever, if
• als = when, but only for a single event in the past
• wann = when? in a question or indirect question

So here:

• wenn sie früh zur Arbeit muss = whenever she has to go to work early

It means has to go to work early.

A useful thing to notice is that German can say:

• Ich muss zur Arbeit = I have to go to work

Even though English often uses go, German does not need to say gehen here. The movement is understood.

So:

• früh = early
• zur Arbeit = to work
• muss = has to

Together: has to go to work early

Zur is a contraction of zu der.

• zu
  • der = zur

The preposition zu takes the dative case, and Arbeit is feminine, so:

• die Arbeit
• zu der Arbeit
• contracted: zur Arbeit

In this expression, zur Arbeit is the normal idiomatic way to say to work.

Here, sie is the subject pronoun inside the subordinate clause, meaning she.

In:

wenn sie früh zur Arbeit muss

the sister is the one who has to go to work early, so she must be the subject.

• sie = she
• ihr can mean her or to her, but not as the subject here

So sie is the correct form.

Most naturally, it sounds habitual or repeated:

Whenever she has to go to work early, my sister cooks oatmeal.

German present tense often covers this kind of general repeated action. Context could make it refer to a present or future situation too, but without extra context, the habitual reading is the most natural one.

Yes:

Wenn sie früh zur Arbeit muss, kocht meine Schwester Haferbrei.

This is completely correct and very common.

Notice what happens in the main clause after the subordinate clause comes first:

• Wenn sie früh zur Arbeit muss, kocht meine Schwester Haferbrei.

The finite verb kocht still comes in the main clause slot immediately after the first element, and the whole wenn clause counts as that first element. So German does not say:

• Wenn sie früh zur Arbeit muss, meine Schwester kocht Haferbrei ❌

It must be:

• Wenn sie früh zur Arbeit muss, kocht meine Schwester Haferbrei ✅