Soror fratrem per vicum sequitur, ne iterum erret.

Because of their cases.

• soror is nominative singular, so it is the subject: the sister
• fratrem is accusative singular, so it is the direct object: the brother

Latin uses case endings much more than word order to show who is doing what.

Because sequitur comes from sequor, sequi, which is a deponent verb.

A deponent verb:

• has passive-looking forms
• but has an active meaning

So:

• sequitur looks like he/she/it is followed
• but actually means he/she/it follows

This is very common in Latin, and sequor is one of the most important deponent verbs to learn.

Sequitur is:

• present tense
• indicative mood
• 3rd person singular

So it means:

• she follows
• he follows
• it follows

In this sentence, because the subject is soror, it means she follows.

Because sequor takes a direct object in the accusative, just like many ordinary active verbs.

So:

• soror fratrem sequitur = the sister follows the brother

Even though sequitur is deponent, it still behaves syntactically like an active transitive verb here.

Because per means through, along, or by way of, and it takes the accusative case.

So:

• per vicum = through the street or along the street

By contrast:

• in vico would usually mean in the street or in the village/neighborhood, depending on context, with more of a location sense than a movement-through sense.

Here the idea is movement along or through the street, so per vicum is the natural choice.

Yes. Vicus can mean things like:

• street
• row of houses
• village
• district/neighborhood

In this sentence, because of per vicum and the overall context of following someone, the meaning through the street or along the street makes the most sense.

Ne introduces a negative purpose clause.

That means it expresses the idea of:

• so that ... not
• in order that ... not
• lest

So:

• ne iterum erret = so that he/she may not wander astray again / lest he/she get lost again

This tells us the purpose of the sister’s action.

Because in Latin, a negative purpose clause normally uses ne, not ut non or just non.

The standard pattern is:

• ut
  • subjunctive = positive purpose
• ne
  • subjunctive = negative purpose

So:

• ut veniat = so that he may come
• ne erret = so that he may not wander astray

Using non here would not be the normal way to form this kind of clause.

Because purpose clauses in Latin take the subjunctive mood.

Since ne iterum erret is a purpose clause, the verb must be subjunctive.

Erret is:

• present subjunctive
• 3rd person singular
• from erro, errare = to wander, go astray, make a mistake

So the clause means something like:

• so that he/she may not wander astray again
• lest he/she get lost again

Iterum means again or a second time.

It modifies erret, so the sense is:

• not wander astray again
• not get lost again

It tells you that this has apparently happened before.

Grammatically, the verb is just 3rd person singular, so the subject is not stated explicitly.

In context, the most natural meaning is usually that it refers to the brother:

• The sister follows her brother through the street so that he will not get lost again.

But Latin does sometimes leave this kind of thing to context. If a writer wanted to make it clearer, they could add an explicit subject, such as:

• ne frater iterum erret

So the sentence as written is understandable, but the exact reference depends somewhat on context.

Not as much as in English.

Latin word order is relatively flexible because the endings show the grammar. The sentence could be rearranged in several ways without changing the basic meaning, for example:

• Soror fratrem per vicum sequitur, ne iterum erret.
• Fratrem soror per vicum sequitur, ne iterum erret.
• Per vicum soror fratrem sequitur, ne iterum erret.

Different orders can give slightly different emphasis, but the cases still show who is subject and object.

Because Latin often omits subject pronouns when the verb ending already tells you the person and number.

For example:

• sequitur already tells you the subject is 3rd person singular
• erret also tells you 3rd person singular

So Latin does not need to say she or he unless there is special emphasis or a need to clarify. This is very normal in Latin.