Mia onklino diris, ke mia kuzino jam preparas vestojn por la ferio.

Because Esperanto usually keeps the tense that matches the actual time of each action.

• diris = said
• preparas = is preparing / prepares

So the sentence means that the aunt spoke in the past, and the cousin’s preparing is presented as a present or ongoing action relative to the speaker’s viewpoint.

This is different from English, which often shifts tenses after a past reporting verb. Esperanto does not have to do that. If the cousin had already finished preparing before the aunt spoke, you would more likely see preparis. If the preparing was still in the future, you could see preparos.

Ke means that and introduces a subordinate clause.

So:

• Mia onklino diris = My aunt said
• ke mia kuzino jam preparas vestojn por la ferio = that my cousin is already preparing clothes for the vacation

In Esperanto, ke is very commonly used in this kind of sentence and is usually not omitted the way English sometimes omits that.

The ending -in- is the feminine suffix in Esperanto.

• onklo = uncle
• onklino = aunt
• kuzo = cousin
• kuzino = female cousin

So onklino and kuzino specifically show that the relatives mentioned are female.

Because vestojn is:

• plural: -j
• direct object: -n

The base word is vesto = a garment / a piece of clothing.

Step by step:

• vesto = one item of clothing
• vestoj = clothes, garments
• vestojn = clothes as the direct object of the verb

Here, the cousin is preparing the clothes, so vestojn is what is being prepared.

Esperanto often leaves that idea unstated when it is not important.

So preparas vestojn can naturally mean:

• is preparing clothes
• is preparing some clothes

English often likes to add some, but Esperanto does not need an extra word here unless the speaker wants to emphasize quantity or indefiniteness.

Jam means already.

It tells you that the action has begun earlier than expected, or that it has happened by this point.

So mia kuzino jam preparas vestojn suggests something like:

• my cousin is already preparing clothes
• she has started doing it now, perhaps earlier than one might have thought

It adds a sense of progress or readiness.

Because por means for, showing purpose or intended use.

• por la ferio = for the vacation / for the holiday
• dum la ferio = during the vacation / during the holiday

In this sentence, the clothes are being prepared in advance for use on that vacation, so por is the natural choice.

La means the, so la ferio refers to a specific vacation or holiday period that is known from the context.

• por ferio would sound more general or less natural here
• por la ferio means for the vacation, a particular one

So the sentence is not talking about vacations in general, but about one specific upcoming vacation.

Not exactly in every context.

Ferio usually means a period when someone is free from work or school, so it often matches English vacation or holiday depending on the variety of English.

For example:

• in American English, vacation is often the closest match
• in British English, holiday is often the closest match

So the exact English translation depends on dialect, but the Esperanto word itself is perfectly normal here.

Because Esperanto normally uses a comma before a subordinate clause introduced by ke.

So the comma separates:

• the main clause: Mia onklino diris
• the subordinate clause: ke mia kuzino jam preparas vestojn por la ferio

This punctuation is standard and helps make the sentence structure clear.

Yes, Esperanto word order is fairly flexible, but the original order is the most neutral and natural.

The sentence uses a very standard pattern:

• subject: Mia onklino
• verb: diris
• subordinate clause: ke...

Inside the subordinate clause, it is also straightforward:

• subject: mia kuzino
• adverb: jam
• verb: preparas
• object: vestojn
• phrase of purpose: por la ferio

Other orders are sometimes possible for emphasis, but this version is the clearest one for normal use.