Η αδερφή μου ξεπαγώνει τα κατεψυγμένα λαχανικά το πρωί, αλλά εγώ ξεπαγώνω το φαγητό το βράδυ.

In Greek, the unstressed possessive pronouns such as μου, σου, του, της usually come after the noun.

So:

• η αδερφή μου = my sister
• το φαγητό μου = my food

This μου literally comes from the genitive form of the pronoun and functions as my here.

A form like μου αδερφή is not the normal basic pattern. If you want extra emphasis, Greek often uses δικός:

• η δική μου αδερφή = my own sister / my sister

Greek normally drops subject pronouns when they are not needed, because the verb ending already shows the person.

• ξεπαγώνει already tells you the subject is he/she/it
• ξεπαγώνω already tells you the subject is I

So the first clause does not need αυτή for she.

In the second clause, εγώ is added for contrast:

• αλλά εγώ... = but I...

That makes the contrast stronger: My sister does X, but I do Y.

They are two forms of the same verb, ξεπαγώνω, meaning to defrost / thaw.

• ξεπαγώνει = he/she/it defrosts
• ξεπαγώνω = I defrost

So in this sentence:

• Η αδερφή μου ξεπαγώνει... = My sister defrosts...
• εγώ ξεπαγώνω... = I defrost...

This is a very common Greek pattern: the verb ending changes depending on the subject.

The prefix ξε- often gives the idea of undoing, removing, or reversing something.

Here:

• παγώνω = to freeze
• ξεπαγώνω = to unfreeze, defrost, thaw

So ξε- often works a bit like English un- or de-, although not in a perfectly one-to-one way in every word.

Because the nouns are different in number and the article has to agree with them.

• λαχανικά = vegetables, neuter plural
• φαγητό = food, neuter singular

So:

• τα κατεψυγμένα λαχανικά = the frozen vegetables
• το φαγητό = the food

Greek articles must agree with the noun in gender, number, and case.

Here both nouns are neuter, but one is plural and one is singular:

• το = neuter singular
• τα = neuter plural

Because the adjective must match the noun it describes.

The noun is:

• λαχανικά = neuter plural

So the adjective also has to be neuter plural:

• κατεψυγμένα = frozen, neuter plural

This is called agreement. In Greek, adjectives agree with nouns in:

• gender
• number
• case

So if the noun changed, the adjective would change too.

In Greek, many common time expressions use the article + noun pattern:

• το πρωί = in the morning
• το βράδυ = in the evening / at night
• το μεσημέρι = at noon / in the afternoon
• τη νύχτα = at night

So the article is not strange here; it is part of the normal expression.

Grammatically, this is an accusative of time, used adverbially to say when something happens.

Most likely it describes a habit or a usual routine.

Greek present tense can mean:

• something happening right now
• something that happens regularly
• a general fact

In this sentence, because of το πρωί and το βράδυ, it sounds like a repeated routine:

• My sister defrosts the vegetables in the morning, but I defrost the food in the evening.

So this is probably habitual present.

Yes, αδελφή also exists and is correct.

• αδερφή is the very common everyday form in Modern Greek
• αδελφή is a more formal or more conservative variant

A learner will very often hear and see αδερφή in ordinary speech and writing. So the form in the sentence is completely natural.

They are in the accusative.

Why?

• τα κατεψυγμένα λαχανικά and το φαγητό are direct objects of the verb ξεπαγώνω
• το πρωί and το βράδυ are accusative time expressions

One thing that can confuse English speakers is that in Greek, neuter nominative and accusative often look the same.

So:

• το φαγητό could be nominative or accusative in form
• τα λαχανικά also look the same in nominative and accusative plural

You have to understand their role from the sentence.

Greek word order is fairly flexible compared with English.

The sentence as given is very natural:

• Η αδερφή μου ξεπαγώνει τα κατεψυγμένα λαχανικά το πρωί, αλλά εγώ ξεπαγώνω το φαγητό το βράδυ.

But Greek can move things around for emphasis or style. For example, a speaker might shift the time expression or emphasize the object.

Even so, the given order is a very standard, neutral order:

• subject
• verb
• object
• time expression

So it is a good model for learners.

Yes. Αλλά means but, and adding εγώ creates a clear contrast.

So αλλά εγώ is not just but I in a neutral sense. It often has the feeling of:

• but I, on the other hand...
• but as for me...

That matches the structure of the sentence very well, because it is comparing two people's routines:

• My sister does it in the morning
• I do it in the evening