Meine Schwester sagt, ihre Haut juckt sofort, wenn eine Wespe sie sticht.

Because German uses commas to separate clauses more consistently than English does. Here, Meine Schwester sagt is the main clause, and ihre Haut juckt sofort, wenn eine Wespe sie sticht is the clause that tells what she says.

There is also a second comma before wenn, because wenn eine Wespe sie sticht is a subordinate clause.

After verbs like sagen, German often allows you to omit dass.

So both of these are possible:

• Meine Schwester sagt, ihre Haut juckt sofort, wenn eine Wespe sie sticht.
• Meine Schwester sagt, dass ihre Haut sofort juckt, wenn eine Wespe sie sticht.

The difference is mainly structure:

• without dass: the following clause keeps normal main-clause word order
• with dass: the verb moves to the end of that clause

So:

• ihre Haut juckt sofort
• dass ihre Haut sofort juckt

Ihre is the possessive adjective meaning her here. It refers to meine Schwester.

So ihre Haut means her skin.

A learner should also know that ihr / ihre / ihren can sometimes mean her or their, depending on context. In this sentence, the context clearly shows it means her, referring to the sister.

Because the possessive word ihre already takes the place of an article.

German normally uses either:

• an article: die Haut
• or a possessive determiner: ihre Haut

You do not usually say die ihre Haut in a sentence like this.

This is because the two clauses are built differently.

In ihre Haut juckt sofort, the clause has main-clause word order, so the finite verb juckt comes in the usual verb-second position.

In wenn eine Wespe sie sticht, wenn introduces a subordinate clause, and in German subordinate clauses usually send the finite verb to the end. That is why sticht comes last.

So:

• main-clause style: ihre Haut juckt sofort
• subordinate clause: wenn eine Wespe sie sticht

Wenn is used for things that happen repeatedly, generally, or whenever something happens.

Here the meaning is something like whenever a wasp stings her or when a wasp stings her in a general sense.

Compare:

• wenn = when / whenever, repeated or general
• als = when, for a single event in the past
• wann = when, in a question

So this sentence uses wenn because it describes a general reaction, not one specific past event.

It refers to meine Schwester.

So the meaning is: when a wasp stings her.

Even though both Wespe and Schwester are feminine nouns, the sentence structure makes it clear:

• eine Wespe = the subject, the one doing the stinging
• sie = the object, the person being stung

Because stechen takes a direct object in the accusative case when you say whom something stings.

So:

• eine Wespe = nominative subject
• sie = accusative object

For a feminine singular pronoun, the accusative form is sie.

Ihr would be dative, and that would not be the normal form with stechen in this sentence.

• eine Wespe is nominative, because it is the subject of sticht
• sie is accusative, because it is the direct object of sticht

This is a useful way to read the clause:

• wer sticht? → eine Wespe
• wen sticht sie? → sie = the sister

German often expresses this idea with the body part as the subject.

So ihre Haut juckt literally means her skin itches, which is very natural German.

English can say her skin itches too, but English also often uses she feels itchy. German tends to prefer the more physical expression with the body part in sentences like this.

Sofort is an adverb, and adverb placement in German is somewhat flexible. In this sentence, ihre Haut juckt sofort sounds natural and straightforward.

It means the itching starts immediately.

If you add dass, many speakers would also naturally say:

• dass ihre Haut sofort juckt

So the exact position can vary a little, but the sentence as given is perfectly normal. The important point is that sofort modifies juckt and tells you when the itching begins.

Yes. A very common alternative is:

Meine Schwester sagt, dass ihre Haut sofort juckt, wenn eine Wespe sie sticht.

That version may feel slightly more explicit or formal to some learners, because dass clearly marks the reported statement. The original version without dass is also completely standard and common in everyday German.