Se o pêssego estiver azedo, minha irmã vai preferir a ameixa.

Because Portuguese often uses the future subjunctive after se when talking about a possible future situation.

So:

• Se o pêssego estiver azedo... = If the peach is sour / if the peach turns out to be sour...

Here, estiver is the future subjunctive form of estar.

This is very common in Portuguese:

• Se ele vier... = If he comes...
• Se chover... = If it rains...
• Se eu tiver tempo... = If I have time...

English usually uses the present after if, but Portuguese often uses the future subjunctive in this kind of sentence.

Estiver is the future subjunctive form of the verb estar.

For estar, the future subjunctive forms are:

• se eu estiver
• se você/ele/ela estiver
• se nós estivermos
• se vocês/eles/elas estiverem

In your sentence:

• Se o pêssego estiver azedo = If the peach is sour

Even though English does not have a special future subjunctive form here, Portuguese does.

Vai preferir is the very common spoken Brazilian Portuguese way to express the future.

• vai preferir = is going to prefer / will prefer
• preferirá = simple future, also correct, but usually more formal or written

So both are possible:

• Se o pêssego estiver azedo, minha irmã vai preferir a ameixa.
• Se o pêssego estiver azedo, minha irmã preferirá a ameixa.

In everyday Brazilian Portuguese, ir + infinitive is extremely common.

Because azedo agrees with o pêssego, which is a masculine noun.

• o pêssego → masculine
• adjective must match → azedo

If the noun were feminine, the adjective would change:

• a ameixa azeda = the sour plum

This is basic adjective agreement in Portuguese:

• masculine singular: azedo
• feminine singular: azeda
• masculine plural: azedos
• feminine plural: azedas

Here, azedo means sour.

For fruit, it usually means the fruit is not sweet and has a sharp, acidic taste.

Depending on context, azedo can also be used more broadly for things that have gone bad, especially with milk or other foods, but in this sentence the most natural meaning is simply sour.

Portuguese uses definite articles much more often than English does.

So:

• o pêssego = the peach
• a ameixa = the plum

Even when English might sound more natural with a bare noun in some contexts, Portuguese often still wants the article.

In this sentence, the article makes the fruit sound specific:

• that/the peach
• that/the plum

Both are possible in Brazilian Portuguese.

• minha irmã
• a minha irmã

Both mean my sister.

In Brazil, the article before a possessive is common, but it is not always required. Usage varies by region, style, and level of formality.

So these are both natural:

• minha irmã vai preferir a ameixa
• a minha irmã vai preferir a ameixa

Because preferir normally takes a direct object in Portuguese.

So:

• preferir a ameixa = to prefer the plum

English learners sometimes expect something like prefer to the plum, but that is not how preferir works here.

You can compare:

• Ela prefere café. = She prefers coffee.
• Ela prefere café a chá. = She prefers coffee to tea.

In your sentence, there is just one thing being preferred, so:

• vai preferir a ameixa

Because this sentence describes a real or possible future condition, not a hypothetical or unlikely situation.

• Se o pêssego estiver azedo, minha irmã vai preferir a ameixa. = If the peach is sour, my sister will prefer the plum.

This is an open condition: it may happen.

Preferiria would usually suggest a more hypothetical or less direct idea, often paired with the imperfect subjunctive:

• Se o pêssego estivesse azedo, minha irmã preferiria a ameixa. = If the peach were sour, my sister would prefer the plum.

So:

• estiver + vai preferir = real possible future
• estivesse + preferiria = more hypothetical

Not in this sentence.

You use estiver because the idea is about the peach’s state or condition being sour.

• estar azedo = to be sour, to taste sour, to be in a sour state

Using ser here would sound wrong, because ser is not normally used for this kind of temporary condition in food.

So:

• correct: o pêssego estiver azedo
• not natural here: o pêssego for azedo

In Brazilian Portuguese, ameixa is usually pronounced roughly like ah-MAY-sha.

A few helpful pronunciation notes:

• a = open ah
• mei sounds like may
• x in ameixa sounds like sh

So:

• ameixa ≈ ah-MAY-sha

Pêssego is pronounced roughly like PEH-sseh-goo in Brazilian Portuguese, with the stress on the first syllable.

The circumflex in pê shows a stressed vowel with a more closed sound:

• pêssego

The main thing for a learner is:

• stress the first syllable: PÊS-se-go

Also, the double ss makes a normal s sound, not a z sound.

Yes, it is natural and grammatically correct.

It sounds like a normal sentence meaning something like:

• If the peach is sour, my sister will prefer the plum.

It is especially natural because it uses:

• future subjunctive after se
• ir + infinitive for the future (vai preferir)

Those are both very common patterns in Brazilian Portuguese.