Mi hermana es argentina.

Because mi is the possessive adjective used before a noun: mi hermana = my sister.

Mía is a possessive pronoun/adjective form used in other structures, such as:

• La hermana es mía = The sister is mine
• Es hermana mía = She is a sister of mine / She’s my sister

So directly before a noun, you use mi, not mía.

Spanish uses ser for things like:

• identity
• origin/nationality
• permanent or defining characteristics

Here, argentina tells us nationality, so Spanish uses ser:

• Mi hermana es argentina.

Using estar here would sound wrong in standard Spanish, because nationality is not treated as a temporary condition.

Because it describes hermana, which is a feminine noun.

In Spanish, adjectives that end in -o / -a usually change to match the gender of the noun:

• argentino for a masculine singular noun
• argentina for a feminine singular noun

So:

• Mi hermano es argentino.
• Mi hermana es argentina.

In Spanish, words for nationalities are normally not capitalized.

So Spanish writes:

• argentina
• mexicano
• colombiana

This is different from English, where you would write Argentinian/Argentine with a capital letter.

However, the country name Argentina is capitalized, because it is a proper noun.

Yes. That means My sister is from Argentina.

There is a small difference:

• Mi hermana es argentina = My sister is Argentinian/Argentine (nationality)
• Mi hermana es de Argentina = My sister is from Argentina (place of origin)

In many situations, they may refer to the same person, but they are not exactly the same idea.

Because in this sentence, argentina is being used as an adjective after ser, not as a noun with an article.

Compare:

• Mi hermana es argentina. = My sister is Argentinian.
• Mi hermana es una argentina famosa. = My sister is a famous Argentine woman.

In the original sentence, no article is needed.

No, that would not sound natural or correct in standard Spanish.

You need something before hermana, such as:

• Mi hermana = my sister
• Tu hermana = your sister
• La hermana = the sister

So Mi hermana es argentina is correct, but Hermana es argentina is not a normal complete sentence.

Because Spanish often drops subject pronouns when the meaning is already clear from the verb or context.

So instead of saying:

• Ella es argentina

you can say:

• Es argentina

And in your full sentence:

• Mi hermana es argentina

the subject is already clearly mi hermana, so ella is unnecessary.

You could say Mi hermana, ella es argentina, but that usually adds emphasis or sounds stylistically marked, not neutral.

The structure is:

possessive + noun + verb + adjective of nationality

Breaking it down:

• Mi = my
• hermana = sister
• es = is
• argentina = Argentinian/Argentine

So the pattern is very similar to English:

• My sister is Argentinian.

A simple Latin American-style pronunciation guide is:

mee ehr-MAH-nah es ahr-hen-TEE-nah

A few helpful notes:

• mi sounds like mee
• hermana has a silent h
• es sounds like ess
• in much of Latin America, the g in argentina before e sounds like a strong English h sound

So argentina often sounds close to:

• ar-hen-TEE-na

Yes, but Mi hermana es argentina is the most neutral and natural order.

Spanish sometimes changes word order for emphasis, for example:

• Argentina es mi hermana would mean The Argentinian one is my sister, which is a completely different idea.
• Argentina mi hermana es is not normal standard word order.

So for a basic statement, keep:

• Mi hermana es argentina

Yes. Hermana means sister, and it is feminine.

The masculine form is hermano = brother.

So:

• Mi hermana es argentina. = My sister is Argentinian.
• Mi hermano es argentino. = My brother is Argentinian.

You would make both the noun and the adjective plural:

• Mis hermanas son argentinas.

Changes:

• mi → mis
• hermana → hermanas
• es → son
• argentina → argentinas

Spanish usually makes all the relevant words agree in number, and adjectives also agree in gender when applicable.