Meine Schwester und ich lesen abwechselnd vor.

Because meine Schwester is the subject of the sentence, so it is in the nominative case.

• meine Schwester = nominative feminine singular
• meiner Schwester is usually dative or genitive

Here, my sister and I are the people doing the action, so German uses the nominative: Meine Schwester und ich.

For the same reason: ich is nominative, and this sentence needs a subject pronoun.

• ich = I
• mich = me

In English, people often say me and my sister in casual speech, but in German the standard form here is meine Schwester und ich, because both parts are subjects.

Because the subject is not just meine Schwester. The full subject is Meine Schwester und ich = my sister and I.

When ich is part of a subject joined by und, the verb usually takes first person plural agreement:

• ich lese = I read
• wir lesen = we read

So:

• Meine Schwester liest = My sister reads
• Meine Schwester und ich lesen = My sister and I read

German main clauses usually follow the verb-second rule: the finite verb goes in the second position.

In this sentence:

• Meine Schwester und ich = first element
• lesen = finite verb in second position

So the structure is:

Meine Schwester und ich | lesen | abwechselnd | vor.

Even though Meine Schwester und ich contains several words, it counts as one unit before the verb.

Because the verb is vorlesen, which is a separable verb.

In a normal present-tense main clause, separable verbs split:

• vorlesen → lesen ... vor

So:

• Ich lese vor.
• Wir lesen abwechselnd vor.

The main part of the verb (lesen) goes in the usual verb position, and the prefix (vor) goes to the end.

lesen means to read.

vorlesen means to read aloud or to read something to someone.

So this sentence is not just about silently reading a book. It means they are taking turns reading aloud.

Examples:

• Ich lese ein Buch. = I am reading a book.
• Ich lese das Buch vor. = I am reading the book aloud.

Here, abwechselnd is an adverb. It describes how they are reading aloud.

It means something like:

• alternately
• taking turns
• one after the other

So abwechselnd tells you that the sister and the speaker are not reading at the same time; they are switching back and forth.

Because abwechselnd is not part of the verb vorlesen. It is just an adverb modifying the verb.

So the structure is:

• verb: vorlesen
• adverb: abwechselnd

That is why German writes:

abwechselnd vorlesen

not as one single verb.

Yes, that is grammatically possible, but Meine Schwester und ich is usually more natural and polite in standard German.

German, like formal English, often puts the speaker last in expressions like this.

So:

• Meine Schwester und ich = more standard/natural
• Ich und meine Schwester = possible, but often less elegant

Because German does not always need to state the object if it is obvious, unimportant, or already known from context.

So Meine Schwester und ich lesen abwechselnd vor is a complete sentence even without saying what they are reading.

You could add an object if you wanted:

• Meine Schwester und ich lesen abwechselnd die Geschichte vor.
  = My sister and I take turns reading the story aloud.

Because all nouns are capitalized in German.

So:

• Schwester = noun → capitalized
• ich = pronoun → not capitalized
• abwechselnd = adverb → not capitalized
• vor = verb prefix here → not capitalized

This is one of the most noticeable spelling rules in German.