Learn Hardcore Portuguese (Brazil): Minha irmã quer ir ao cinema, mas meu irmão prefere o museu. - My sister wants to go to the movie theater, but my brother prefers the museum.

ir ao cinema – ir (to go) normally takes a preposition indicating direction (a → ao).
preferir o museu – preferir (to prefer) takes a direct object with no preposition.

So:

quer ir ao cinema – wants to go to the movies
prefere o museu – prefers the museum

If you want to keep the same structure with ir, you can say:

Meu irmão prefere ir ao museu. – My brother prefers to go to the museum.

What’s the difference between ao cinema, para o cinema and no cinema?

All three are common, but they are used a bit differently:

ao cinema = to the cinema (focus on movement toward a place)
para o cinema = to the cinema (also movement/destination; very frequent in spoken Brazilian Portuguese)
no cinema = in/at the cinema (location, not movement)

Examples:

Hoje eu vou ao cinema. – Today I’m going to the movies.
Hoje eu vou para o cinema. – Same idea, very natural in Brazil.
Hoje eu estou no cinema. – Today I’m at the movies (already there).

In your sentence, quer ir ao cinema and quer ir para o cinema are both possible; no cinema would sound like “wants to be at the cinema,” not “go to the cinema.”

Why is there no article before minha irmã and meu irmão? I often see a minha irmã.

Both patterns exist, but there is a difference in variety and style:

In Brazilian Portuguese, it’s very common (and usually more natural) to omit the article with singular close family members:
- minha irmã, meu irmão, minha mãe, meu pai
In European Portuguese, you very often see the article:
- a minha irmã, o meu irmão

In Brazil you can say a minha irmã and o meu irmão, but in everyday speech many people simply say minha irmã, meu irmão as in your sentence.

Could I say Meu irmão prefere ir ao museu instead of prefere o museu? Does it change the meaning?

Yes, you can, and the meaning is almost the same.

prefere o museu – “prefers the museum” (the place or the idea of visiting it)
prefere ir ao museu – “prefers to go to the museum” (makes the action explicit)

In context (cinema vs museum), both are understood as a preference between the two activities. The version in your sentence is shorter and completely natural.

Can I use porém or só que instead of mas? Are they all “but”?

All three can express contrast, but they differ in tone and position:

mas – the most common and neutral “but”:
- …ao cinema, mas meu irmão prefere o museu.
porém – more formal/literary, often used after a comma, sometimes in the middle of the sentence:
- …ao cinema, porém meu irmão prefere o museu.
só que – very colloquial, like “it’s just that/but” in speech:
- …ao cinema, só que meu irmão prefere o museu.

In everyday spoken Brazilian Portuguese, mas and só que are much more common than porém.

How are irmã and irmão pronounced? What does the tilde (~) mean?

The tilde (~) over ã and ão marks nasal vowels.

irmã: roughly eer-MAHN
- final ã is a nasal a (like saying “ah” while letting air go through your nose)
irmão: roughly eer-MOWN (not exactly English “own”)
- ão is a nasal sound, somewhere between “ow” and “ong,” but nasal.

Other tips:

The r in irmã/irmão in Brazilian Portuguese is often a soft, breathy sound (similar to the h in English “house”) depending on the region.
The final m is not fully pronounced; it mostly just makes the vowel nasal.

Why does mas end in s but often sound like “maz”?

In Brazilian Portuguese, s between vowels is usually pronounced like z.

In mas meu irmão:

You say it as one flow: masmeu…
The s of mas is between the vowel a and the vowel e of meu.
So it sounds like maz meu.

Compare:

mas (but) – maz before a vowel
mais (more) – majs, with an s sound at the end

Do irmã and irmão change form in the plural?

Yes, they do:

irmã → irmãs (sisters)
irmão → irmãos (brothers / siblings)

Patterns:

many feminine nouns ending in -ã → -ãs in the plural:
- maçã → maçãs (apple[s])
many masculine nouns ending in -ão → -ões or -ãos:
- irmão → irmãos
- leão → leões (lion[s])

So:

Minhas irmãs querem ir ao cinema.
Meus irmãos preferem o museu.

Can I drop meu irmão the second time and just say Minha irmã quer ir ao cinema, mas prefere o museu?

Grammatically, yes; Portuguese allows subject drop because the verb form often shows who the subject is.

However, in your exact sentence it becomes ambiguous:

mas prefere o museu could mean:
- she prefers the museum (the sister), or
- he prefers the museum (the brother), depending on earlier context.

If both irmã and irmão have just been mentioned, it’s clearer to repeat:

…, mas meu irmão prefere o museu.

If the subject is already crystal clear from context, you can omit it:

Meu irmão não gosta de cinema. Prefere o museu. – Here it’s clear we’re still talking about my brother.