Mia onklino alvenos morgaŭ, kaj mia onklo venos kun ŝi per trajno.

The ending -os marks the future tense in Esperanto.

So:

• alvenos = will arrive
• venos = will come

Esperanto tense endings are very regular:

• -as = present
• -is = past
• -os = future

That means if you know the verb stem, you can immediately tell the time from the ending.

They are related, but not identical.

• veni means to come
• alveni means to arrive

The prefix al- often gives the idea of to, toward, up to a destination. So alveni focuses on reaching the destination, while veni focuses more generally on coming.

In this sentence:

• Mia onklino alvenos morgaŭ = your aunt will arrive tomorrow
• mia onklo venos kun ŝi = your uncle will come with her

So the aunt’s arrival is emphasized in the first clause, while the second clause describes the uncle coming along with her.

Onklo means uncle, and onklino means aunt.

The key part is the suffix -in-, which marks the feminine form.

• onklo = uncle
• onklino = aunt

This pattern is very common in Esperanto:

• patro = father
• patrino = mother
• frato = brother
• fratino = sister

So if you already know onklo, then onklino is easy to recognize.

Mia means my.

It appears twice because both nouns belong to the speaker:

• mia onklino = my aunt
• mia onklo = my uncle

Yes, mia can change, because it behaves like an adjective and can agree with the noun when needed:

• mia onklo = my uncle
• miaj onkloj = my uncles
• mian onklon = my uncle, as a direct object
• miajn onklojn = my uncles, as direct objects

In your sentence, both nouns are singular and not direct objects, so plain mia is correct.

In Esperanto, you normally do not use la with possessives like mia, via, lia, and so on.

So you say:

• mia onklino
• via libro
• ŝia domo

not usually:

• la mia onklino
• la via libro

Using mia already makes the noun definite enough, just like my aunt in English does not need the.

Morgaŭ means tomorrow.

Here it works as an adverb of time, telling you when the aunt will arrive:

• Mia onklino alvenos morgaŭ = My aunt will arrive tomorrow

Esperanto adverbs often answer questions like:

• when?
• how?
• where?

Also, word order is flexible, so you could also say:

• Morgaŭ mia onklino alvenos

That would still mean the same thing.

Esperanto does not make the same she/her distinction that English does.

The basic pronoun is:

• ŝi = she / her

Its grammatical role is shown by the sentence structure and endings, not by switching to a completely different word.

After a preposition like kun meaning with, you use the normal pronoun form:

• kun ŝi = with her

If ŝi were a direct object, it could take -n:

• Mi vidas ŝin = I see her

But after kun, you do not use ŝin here. So kun ŝi is correct.

Per means by means of or using.

So per trajno literally means something like by means of a train, which in natural English becomes by train.

This is a very common Esperanto pattern for means or method:

• per aŭto = by car
• per buso = by bus
• per telefono = by phone
• per trajno = by train

This is different from kun:

• kun ŝi = with her
• per trajno = by train

So one phrase shows company, and the other shows means of travel.

You could say en trajno, but it emphasizes being inside a train rather than the means of travel.

The usual way to say by train is per trajno.

Compare:

• Mi veturas per trajno = I travel by train
• Mi estas en trajno = I am in a train

So in your sentence, per trajno is the natural choice because the point is the method of travel.

Because there is no direct object here.

The main nouns are subjects:

• Mia onklino = subject of alvenos
• mia onklo = subject of venos

And the other noun phrases come after prepositions:

• kun ŝi
• per trajno

Normally, nouns and pronouns after prepositions do not take -n.

So all of these are exactly what you would expect in this sentence.

In Esperanto, each finite verb normally shows its own tense.

So even though the whole sentence is about the future, both verbs stay in the future form:

• alvenos
• venos

That is why the sentence says:

• Mia onklino alvenos morgaŭ, kaj mia onklo venos kun ŝi per trajno.

This is normal Esperanto style. Each clause marks its own verb clearly.

Yes. Esperanto word order is fairly flexible because grammatical roles are shown clearly by endings and prepositions.

For example, you could also say:

• Morgaŭ mia onklino alvenos, kaj mia onklo venos kun ŝi per trajno.

That still means the same thing.

However, the original order is very natural and straightforward. Learners will usually see this kind of word order first because it is clear and easy to follow.

A rough guide for an English speaker:

• ŝi sounds like shee
• trajno sounds roughly like TRY-no
• morgaŭ sounds roughly like mor-GOW

A couple of useful points:

• ŝ is pronounced like English sh
• j is pronounced like English y
• aŭ sounds like the ow in cow

So:

• trajno has j = y sound
• morgaŭ ends with the diphthong aŭ

Esperanto pronunciation is very regular, so once you learn the sound of each letter, reading gets much easier.