Learn Hardcore Portuguese (Portugal): A Ana toca guitarra e o irmão toca bateria numa banda. - Ana plays the guitar and her brother plays the drums in a band.

In Portugal this sounds very natural in everyday speech and a lot of writing.
You can also say Ana toca guitarra… without the article; that’s a bit more neutral/formal.
When you directly address someone, you normally drop the article: Ana, vem cá! (not A Ana, vem cá!).

Why is it toca guitarra and not toca a guitarra? In English we say plays the guitar.

With musical instruments, Portuguese can use the definite article, but very often it simply omits it:

tocar guitarra – to play (the) guitar, in general
tocar a guitarra – to play the guitar (more specific, a particular guitar, or a bit more formal)

In this sentence, toca guitarra talks about the skill or activity in general, not about a specific physical guitar, so dropping the article is perfectly natural. In European Portuguese you will hear both patterns, but the article is not required the way the is in English.

What is the difference between toca and tocar?

Tocar is the infinitive, to play (an instrument), to touch, to ring, etc.

Toca is the present tense, third person singular form:

ele/ela toca = he/she plays, he/she touches, it rings

So:

A Ana toca guitarra → Ana plays guitar
Ela gosta de tocar guitarra → She likes to play guitar

Same verb, different form.

Does toca always mean plays, or can it also mean touches?

Tocar is a multipurpose verb. Depending on context it can mean:

to play (an instrument): Ela toca piano.
to touch: Não toques no vidro. (Don’t touch the glass.)
to ring / go off: O despertador toca às sete. (The alarm rings at seven.)
to knock (on a door): Alguém está a tocar à porta.

In your sentence, because it’s followed by musical instruments (guitarra, bateria), it clearly means plays.

How do we know that o irmão means her brother here and not just the brother of someone else?

Portuguese can drop the possessive when it is obvious from context, especially with family words:

A Ana toca guitarra. O irmão toca bateria.

Because Ana has just been mentioned, o irmão is naturally understood as Ana’s brother.

If you said this out of the blue with no context, o irmão would be ambiguous, but inside a mini-story like this, listeners assume it means her brother.

Can I say o irmão dela or o seu irmão instead of just o irmão?

Yes, both are possible, but each has a nuance:

o irmão dela – her brother (literally the brother of her). Very clear and explicit.
o seu irmão – can mean her brother or your brother, depending on context. In European Portuguese it is still quite common, but it can be ambiguous.
o irmão (as in the sentence) – her brother, understood from context.

In careful or learner Portuguese, o irmão dela is the safest way to be 100% explicit:
A Ana toca guitarra e o irmão dela toca bateria numa banda.

Why is toca repeated? Could I say A Ana toca guitarra e o irmão bateria?

You normally repeat the verb in Portuguese:

A Ana toca guitarra e o irmão toca bateria.

Leaving it out:

A Ana toca guitarra e o irmão bateria

sounds incomplete or marked. Native speakers might occasionally drop the second toca in very informal, fast speech, but in standard written or careful spoken Portuguese you repeat it for clarity and naturalness.

What exactly does bateria mean here?

In this sentence, bateria means the drums / a drum kit, i.e. the percussion instrument in a band.

Be careful: bateria is a false friend:

bateria can mean battery (for cars, devices) in other contexts.
But when you see it with tocar as a musical activity (tocar bateria), it means to play the drums.

What is numa and how is it different from em uma, na, or em uma banda?

Numa is a contraction:

em + uma → numa

So:

em uma banda and numa banda mean the same: in a band.
In normal speech and writing, the contracted form numa is much more common and natural.

Compare:

numa banda → in a band (indefinite; we don’t care which band)
na banda (em + a banda) → in the band (a specific, known band)

Can I move numa banda to the beginning: Numa banda, a Ana toca guitarra e o irmão toca bateria?

Yes. That word order is perfectly correct:

A Ana toca guitarra e o irmão toca bateria numa banda.
Numa banda, a Ana toca guitarra e o irmão toca bateria.

Moving numa banda to the front just emphasizes the setting in a band a bit more, but the basic meaning is the same. Portuguese word order is fairly flexible for phrases like this.

Does the present tense toca mean they are playing right now, or that they generally play in a band?

Here, toca is a habitual present: it describes something that is generally true:

A Ana toca guitarra → Ana plays guitar (as a skill, as an activity she does).

If you want to emphasize right now in European Portuguese, you usually use:

está a tocar → is playing (right now)
A Ana está a tocar guitarra.

But in headlines, stories, or simple statements, the simple present toca often covers the English ideas of plays and sometimes is playing.

How would the sentence change if the sibling was a sister instead of a brother?

You only need to change o irmão (brother) to a irmã (sister):

A Ana toca guitarra e a irmã toca bateria numa banda.

Some related forms:

irmão – brother
irmã – sister
irmãos – brothers / siblings (mixed group)
irmãs – sisters

In Portugal, is guitarra the same as violão in Brazilian Portuguese?

Roughly, yes:

In Brazil: violão usually = acoustic guitar, guitarra = electric guitar.
In Portugal: guitarra is the normal word for guitar in general (acoustic or electric), unless you specify:
- guitarra acústica – acoustic guitar
- guitarra elétrica – electric guitar

So in European Portuguese, A Ana toca guitarra just means Ana plays guitar, without specifying acoustic or electric.

How do you pronounce irmão, especially the ão sound?

In European Portuguese, irmão is roughly:

ir-MÃO

Details:

ir – like eer in beer, but shorter.
mão – nasal sound, a bit like mown in English mown, but with air through the nose and no clear final n.

The ending ão is a nasal diphthong. You’ll hear it in many words:

não (no)
pão (bread)
coração (heart)

Listening to native audio is very helpful for getting ão right.