La meva germana prepara la mudança i es muda demà.

In standard Catalan, it is very normal to use the definite article before a possessive:

• la meva germana = my sister
• el meu pare = my father
• la nostra casa = our house

So la meva germana is the ordinary, natural way to say my sister. Leaving out la is not the neutral default in modern standard Catalan.

Both are 3rd person singular present indicative forms, matching the subject la meva germana.

• prepara comes from preparar = to prepare
• es muda comes from mudar-se = to move house

So literally:

• prepara = she prepares
• es muda = she moves / is moving house

Because the verb here is mudar-se, not just mudar.

In Catalan, mudar-se is the usual verb for to move house. The es is the 3rd person singular reflexive/pronominal pronoun:

• jo em mudo = I move
• tu et mudes = you move
• ella es muda = she moves

So es is not optional here. It is part of the verb expression mudar-se.

They are related, but one is a noun and the other is a verb:

• la mudança = the move, the house move, the relocation
• es muda = she moves / she is moving house

It is similar to English:

• the move = noun
• she moves = verb

So the sentence uses both:

• first the thing being prepared: la mudança
• then the action itself: es muda

Catalan, like English, often uses the present tense for a planned or near-future event.

So:

• es muda demà = she moves tomorrow / she is moving tomorrow

This is completely natural. If you wanted to make the future more explicit, you could also say:

• es mudarà demà = she will move tomorrow

But the present tense is very common for scheduled or intended future actions.

Yes. Catalan commonly uses preparar directly with the thing being prepared.

So:

• prepara la mudança = prepares the move / gets the move ready / organizes the move

In English, we often say prepare for the move, but Catalan does not need that extra preposition here.

Not if you mean moves house.

• es muda = she moves house
• muda without es would usually suggest a different meaning of mudar, not the normal move house meaning

So for relocation, use mudar-se:

• La meva germana es muda demà.

Yes, Catalan word order is flexible, and demà can appear in different places.

The version in your sentence is very natural:

• La meva germana prepara la mudança i es muda demà.

You could also say:

• Demà la meva germana prepara la mudança i es muda.

Putting demà at the beginning can make it feel like a time frame for the whole sentence. At the end, it most directly attaches to the second verb, though context can broaden it.

The ç is called ce trencada in Catalan. It makes an s sound before a, o, or u.

So:

• mudança is pronounced with an s sound, not a k sound

A rough English-friendly pronunciation would be:

• moo-DAN-sa

And in demà, the accent mark shows that the stress is on the last syllable:

• de-MA

Yes, muda can have other meanings in other contexts, which can confuse learners.

For example, mut/muda can mean mute as an adjective. But in this sentence, es muda is clearly a verb form from mudar-se.

The context makes it clear:

• prepara la mudança = prepares the move
• es muda demà = moves tomorrow

So here there is no ambiguity once you see the whole sentence.