Meine Schwester isst lieber veganes Müsli mit Blaubeeren und Himbeeren.

Because Schwester is a feminine noun: die Schwester.

The possessive word mein- changes its ending to match the noun’s gender, number, and case. Here, Schwester is:

• feminine
• singular
• nominative, because it is the subject of the sentence

So the correct form is meine Schwester.

Compare:

• mein Bruder = my brother
• meine Schwester = my sister

The verb is essen = to eat. In this sentence, it has to be conjugated for meine Schwester, which is third-person singular, like she in English.

So:

• ich esse = I eat
• du isst = you eat
• er/sie/es isst = he/she/it eats
• wir essen = we eat
• ihr esst = you all eat
• sie essen = they eat

Since meine Schwester means she, the correct form is isst.

Because essen is a stem-changing verb in the present tense.

In the du and er/sie/es forms, the vowel e changes to i:

• du isst
• er isst
• sie isst

That is just part of the verb’s pattern and has to be learned with the verb.

Lieber expresses preference. In this sentence, it means something like:

• prefers
• would rather eat
• likes better

So Meine Schwester isst lieber veganes Müsli ... means that vegan muesli is the option she prefers.

A useful comparison:

• gern = gladly, likes doing
• lieber = rather, prefers
• am liebsten = most of all, likes best

Examples:

• Sie isst gern Müsli. = She likes eating muesli.
• Sie isst lieber Müsli. = She prefers eating muesli.
• Sie isst am liebsten Müsli. = Muesli is what she likes best.

Because German main clauses usually place the finite verb in second position, and words like lieber often appear in the middle of the sentence before the direct object.

The structure here is:

• Meine Schwester = subject
• isst = finite verb
• lieber = adverb expressing preference
• veganes Müsli = direct object
• mit Blaubeeren und Himbeeren = prepositional phrase

So the word order is very normal for German.

Because Müsli is a neuter noun: das Müsli.

It is also the direct object of isst, so it is in the accusative case. With no article in front of it, the adjective takes a strong ending.

For neuter singular accusative, that ending is -es:

• veganes Müsli

So:

• gender: neuter
• number: singular
• case: accusative
• adjective ending: -es

German often leaves out the article when talking generally about food, drink, or substances.

So Sie isst veganes Müsli sounds natural and general, similar to:

• Er trinkt Kaffee.
• Sie isst Brot.

You could also say:

• ein veganes Müsli if you mean one serving or one bowl
• das vegane Müsli if you mean a specific vegan muesli already known in the conversation

Because mit always takes the dative case.

Here, Blaubeeren and Himbeeren are plural nouns. In the dative plural, German often adds -n, but these words already end in -n, so they do not change further.

So:

• mit Blaubeeren
• mit Himbeeren

That is the correct dative plural form after mit.

Because the sentence is talking about muesli with blueberries and raspberries as ingredients, not with a single blueberry or a single raspberry.

In both English and German, ingredients like this are normally plural:

• mit Blaubeeren
• mit Himbeeren

If you wanted to talk about just one berry, you would use the singular, but that would sound unusual here.

In everyday German, Müsli is usually treated as a singular mass noun, much like cereal in English.

So:

• das Müsli = the muesli
• veganes Müsli = vegan muesli

Sometimes you may see Müslis in advertising or on packaging to mean different types of muesli, but for basic everyday use, think of Müsli as singular.

Because in German, all nouns are capitalized.

That is a standard spelling rule, not something special about this sentence.

So in this sentence:

• Schwester is a noun
• Müsli is a noun
• Blaubeeren is a noun
• Himbeeren is a noun

That is why they all begin with capital letters.

It is a standard German main clause with the finite verb in second position.

You can break it down like this:

• Meine Schwester = subject
• isst = verb
• lieber = adverb of preference
• veganes Müsli = direct object
• mit Blaubeeren und Himbeeren = prepositional phrase

So the sentence follows a very common German pattern: subject + finite verb + middle elements + object/details