Se la vetero restos bona, ni flugos al la urbo, kie loĝas mia fratino, kaj vidos la riveron tra la fenestro.

Se means if. It introduces a condition:

• Se la vetero restos bona... = If the weather stays good...

This is a very common way to form conditional sentences in Esperanto.

Restos has the future ending -os, so it means will remain / will stay.

• restas = stays / is staying
• restos = will stay

Since the sentence talks about a future possibility, Esperanto uses the future tense in both parts:

• Se la vetero restos bona, ni flugos...
• If the weather stays good, we will fly...

Unlike English, Esperanto often keeps the future tense in the if-clause when the meaning is truly future.

Because bona describes la vetero and is not a direct object.

In Esperanto, adjectives agree with the nouns they describe in number and case:

• vetero is singular and not accusative
• so the adjective is also singular and not accusative: bona

If it were a direct object, you might see bonan, but here it is part of the predicate after restos:

• la vetero restos bona = the weather will remain good

Because al shows direction to a place.

• flugos al la urbo = will fly to the city

Esperanto often uses al for movement toward something. The accusative -n can also sometimes show direction, but with verbs like flugi, using al is very natural and clear.

So here:

• al la urbo = to the city

This is a relative clause describing la urbo.

• la urbo = the city
• kie = where
• loĝas mia fratino = my sister lives

So:

• la urbo, kie loĝas mia fratino = the city where my sister lives

The clause gives extra information about the city.

Because kie means where, and the sentence is talking about a place.

• kie = where
• kiu = who / which

Since urbo is a place, Esperanto uses kie:

• la urbo, kie loĝas mia fratino = the city where my sister lives

You could think of kie as meaning something like in which place.

Both are possible.

Esperanto word order is flexible because the grammar is marked clearly by endings. So these both work:

• kie loĝas mia fratino
• kie mia fratino loĝas

The version in the sentence may sound a bit smoother or slightly more literary, but it is not strange. The subject is still mia fratino, because loĝi is intransitive and there is no ambiguity.

Loĝi means to live / reside, and with kie you do not need tie or en.

• kie loĝas mia fratino literally means where my sister lives

Because kie already contains the idea of place, adding en would usually be unnecessary here.

You could say:

• la urbo, en kiu loĝas mia fratino

That also means the city in which my sister lives, but it is a little more formal. Kie is simpler and very common.

Because the subject is still the same: ni.

The sentence says:

• ni flugos ... kaj vidos ...

That means:

• we will fly ... and [we will] see ...

Esperanto often leaves out the subject when it is the same and clearly understood from the previous verb.

Because la riveron is the direct object of vidos.

• vidi = to see
• what will we see? la riveron

In Esperanto, direct objects take -n:

• la rivero = the river
• la riveron = the river (as a direct object)

So:

• vidos la riveron = will see the river

Because tra means through, while el means out of / from inside.

• tra la fenestro = through the window
• el la fenestro = out of the window

If you are looking and seeing something on the other side of the glass, tra la fenestro is the natural choice.

The commas separate an inserted descriptive clause.

Main structure:

• Se la vetero restos bona, ni flugos al la urbo ... kaj vidos la riveron tra la fenestro.

Inserted description:

• kie loĝas mia fratino

So the commas help show that this part adds information about la urbo.

Also, Esperanto often uses commas a bit more regularly than English, especially before or after subordinate clauses.

La is the definite article, meaning the.

Esperanto uses la when the speaker means a specific thing:

• la vetero = the weather
• la urbo = the city
• la riveron = the river
• la fenestro = the window

In this sentence, each noun is treated as specific or identifiable in context. Esperanto has only one definite article: la. It does not change for gender, number, or case.

Yes. Bona is a broad adjective meaning good, but with weather it can often be understood as:

• good
• nice
• fine

And resti can mean:

• remain
• stay

So la vetero restos bona could be understood in several very natural English ways, such as:

• the weather will stay good
• the weather will remain nice
• the weather will stay fine

The exact English wording may vary, but the Esperanto grammar stays the same.