Learn Hardcore Portuguese (Brazil): Minha irmã mais nova não sabe nadar, então ela só senta perto da piscina e lê. - My younger sister doesn’t know how to swim, so she just sits near the pool and reads.

irmã mais nova – younger/youngest sister (very common and neutral)
irmã caçula – also “youngest sister”, very common in Brazil, a bit more informal/affectionate

Nova/jovem as adjectives for people can both mean “young”, but irmã mais jovem is less common in everyday speech than irmã mais nova or irmã caçula.

Why is the adjective order irmã mais nova (noun + adjective) instead of mais nova irmã?

In Portuguese, adjectives usually come after the noun: irmã nova, casa bonita, carro velho.
So irmã mais nova (literally “sister more young”) follows the normal pattern: noun + (mais) + adjective.
Placing the adjective before the noun (nova irmã) is possible in some cases, but it usually changes the nuance (e.g., minha nova irmã might suggest “my new (recently acquired/arrived) sister”, like a stepsister or sister-in-law).

Why is it não sabe nadar and not não pode nadar or não nada?

saber + infinitive (here sabe nadar) means to know how to do something → não sabe nadar = “doesn’t know how to swim.”
poder + infinitive (não pode nadar) means “cannot / is not allowed / is not able to swim” (for some reason: rules, health, circumstances).
não nada would literally be “does not swim” (habitual action), not “doesn’t know how to swim.”

So não sabe nadar is the natural way to say “doesn’t know how to swim.”

What is the function of então here, and could I use por isso or assim instead?

Então here is a conjunction meaning so / therefore, introducing a consequence.
You could also say:

…não sabe nadar, por isso ela só senta… – very close in meaning, slightly more formal/explicit (“for that reason”).
…não sabe nadar, então ela só senta… – very common, neutral speech.
…não sabe nadar, assim ela só senta… – possible in writing, but assim in this sense is less common in everyday speech.

In conversation, então and por isso are the most natural options for this sentence.

What exactly does só mean in ela só senta perto da piscina? Does it mean “only” or “alone”?

Here só means only. So ela só senta perto da piscina = “she only sits near the pool.”
Só can mean:

only (limiting the action):
- Ela só senta perto da piscina. – That’s the only thing she does.
alone (when used differently in the sentence or with context):
- Ela senta só. – She sits alone. (but even here, context matters)

In your sentence, because só is before the verb, it’s clearly understood as only.

Could I move só in the sentence? What would change if I said Ela senta só perto da piscina?

Yes, moving só slightly changes what is being limited:

Ela só senta perto da piscina.
→ The only thing she does is sit near the pool (she doesn’t swim, play, etc.).
Ela senta só perto da piscina.
→ She sits only near the pool (she doesn’t sit in other places).

Both are grammatically fine but emphasize different things. The original emphasizes the type of activity; the second emphasizes the location.

Why is it ela só senta and not ela só se senta? When do you use sentar vs sentar-se?

Sentar can be used with or without the reflexive pronoun se in Brazilian Portuguese:

Ela senta.
Ela se senta.

Both are correct and mean “she sits down.” In everyday Brazilian speech, the non-reflexive form (sentar) is more common, especially with a place phrase: senta aqui, senta ali, senta perto da piscina.
The reflexive form sentar-se is more formal or more common in Portugal.

What does perto da piscina literally mean, and why do we say da and not de a?

Perto da piscina literally means near the pool.

perto de = near (something)
a piscina = the pool

In Portuguese, de + a contract to da:

perto de + a piscina → perto da piscina

So da is just the contraction of de and a, and it’s mandatory in standard Portuguese when those two prepositions meet.

Could I say perto de piscina without the article, or is perto da piscina better?

In Brazilian Portuguese, with concrete, countable nouns like piscina, you almost always use the definite article after perto de when you are talking about a specific thing:

perto da piscina – near the (the usual / that) pool

Perto de piscina sounds unnatural unless it’s a very unusual, abstract context (e.g., in a rule like Nada de correr perto de piscina – even then, most Brazilians would still say da piscina).
So perto da piscina is the normal, natural form.

Why is it lê at the end and not ler or something like lei?

Ler is the infinitive “to read.” The subject is ela (she), so we use the 3rd person singular of the present tense:

eu leio – I read
você/ele/ela lê – you/he/she reads
nós lemos – we read
eles/elas leem – they read

So ela lê = “she reads.”
Lei is a noun meaning law, not a verb form, so it would be wrong here.

Why do we use simple present (senta… lê) instead of a continuous form like está sentando… está lendo?

Portuguese uses the simple present much more than English to talk about:

habitual actions: Ela só senta perto da piscina e lê. – “She (usually) just sits near the pool and reads.”
actions happening “around now” in a general way.

If you say Ela está sentando perto da piscina e está lendo, it focuses on what she is doing right now at this moment, and even then está sentando is unusual; people would normally say está sentada (is seated) + está lendo (is reading).

For a neutral, general description of what she does, the simple present senta… lê is the most natural.

Do you really need the subject pronoun ela here, or could you say …então só senta perto da piscina e lê?

You can omit the subject pronoun in Portuguese because the verb ending shows the subject. So:

Então ela só senta perto da piscina e lê.
Então só senta perto da piscina e lê.

Both are grammatically correct.
However, Brazilians often keep the pronoun ela to make it clearer and more natural in continuous discourse, especially at the beginning of a clause. In your sentence, ela sounds very natural and helps link back clearly to minha irmã mais nova.

Is there anything special about the accent in irmã?

Yes. Irmã has a tilde (ã), which indicates a nasal vowel and also shows the stressed syllable: ir-MÃ.
Pronunciation is roughly like ir-MAHN (with nasal airflow on the last vowel).
Without the tilde (irma), the word would be spelled wrong and pronounced differently, so the accent is essential.