Meine Cousine isst vegan, aber ihre Schwester kocht nur manchmal vegetarisch.

Because meine and ihre are possessive words, like my and her in English.

Here they both describe feminine singular nouns in the nominative case:

• die Cousine → meine Cousine
• die Schwester → ihre Schwester

The ending -e matches that form.

Compare:

• mein Cousin = my male cousin
• meine Cousine = my female cousin
• ihr Bruder = her brother
• ihre Schwester = her sister

German usually distinguishes male and female cousins:

• der Cousin = male cousin
• die Cousine = female cousin

English uses cousin for both, but German often makes the gender clear in the noun itself.

Because in this sentence, vegan and vegetarisch are being used adverbially, not directly before a noun.

So:

• vegan essen = to eat vegan / to eat a vegan diet
• vegetarisch kochen = to cook vegetarian food

There is no noun right after them, so they do not take adjective endings here.

Compare:

• Sie isst vegan. = She eats vegan.

• Sie isst veganes Essen. = She eats vegan food.

• Er kocht vegetarisch. = He cooks vegetarian.
• Er kocht vegetarische Gerichte. = He cooks vegetarian dishes.

Because kochen and essen express different ideas.

• isst vegan means your cousin’s eating habits are vegan.
• kocht nur manchmal vegetarisch means her sister only cooks vegetarian food sometimes.

This does not necessarily mean the sister herself is vegetarian. It only tells you something about what she cooks, and how often.

If you said:

• Ihre Schwester isst nur manchmal vegetarisch, that would mean she only eats vegetarian food sometimes.

So the choice of verb changes the meaning quite a bit.

Nur manchmal means only sometimes.

• manchmal = sometimes
• nur = only

Together, they limit the frequency:

• kocht nur manchmal vegetarisch = cooks vegetarian only sometimes

So the sister does it occasionally, not regularly.

In a normal German main clause, the finite verb usually stays in second position.

So in:

• ... aber ihre Schwester kocht nur manchmal vegetarisch

the structure is:

1. ihre Schwester = subject
2. kocht = verb
3. nur manchmal vegetarisch = the rest of the sentence

That is very normal German word order.

You could move parts around for emphasis, but the version in the sentence is the most natural neutral order.

Because aber is a coordinating conjunction, like but in English.

It connects two main clauses, so both clauses keep normal main-clause word order:

• Meine Cousine isst vegan
• aber ihre Schwester kocht nur manchmal vegetarisch

The verb stays in second position in both parts.

Compare that with a subordinating conjunction like weil:

• ..., weil ihre Schwester nur manchmal vegetarisch kocht.

There, the verb kocht goes to the end.

Yes, ihre can sometimes mean different things depending on context, such as:

• her
• their
• sometimes even formal your

But in this sentence, the natural meaning is her sister, referring back to meine Cousine.

So:

• Meine Cousine ... aber ihre Schwester ... most naturally means:
• My cousin ... but her sister ...

Context usually makes this clear.

Because all nouns are capitalized in German.

So in this sentence:

• Cousine is a noun
• Schwester is a noun

That is why they begin with capital letters.

This is a basic rule in German, unlike English, where common nouns are usually not capitalized.

It is talking about a general habit or usual behavior.

• Meine Cousine isst vegan = My cousin eats vegan / follows a vegan diet
• ihre Schwester kocht nur manchmal vegetarisch = her sister only sometimes cooks vegetarian

German present tense often covers both:

• what is happening now
• what usually happens

In this sentence, the meaning is clearly habitual, not just about one specific meal right now.