Mi hermano es estadounidense.

Because mi is the possessive adjective used before a noun:

• mi hermano = my brother

Mío is usually used differently, often on its own or after the noun:

• El hermano mío = my brother
• Ese hermano es mío = That brother is mine

So in this sentence, mi hermano is the normal and natural form.

In Spanish, a possessive like mi, tu, su, nuestro, etc. usually replaces the article.

So you say:

• mi hermano
• not el mi hermano

That old-style structure existed historically, but in modern standard Spanish, mi hermano is the normal form.

Es is the third-person singular present tense of ser.

So:

• yo soy
• tú eres
• él/ella/usted es

Since mi hermano is he / my brother, Spanish uses es.

Because nationality is normally expressed with ser, not estar.

Spanish usually uses ser for things like:

• identity
• origin
• nationality
• profession
• basic characteristics

So:

• Mi hermano es estadounidense. = correct

But:

• Mi hermano está estadounidense. = not correct for nationality

A learner shortcut is: for who someone is or where someone is from, use ser.

In Latin America, estadounidense is often preferred because it is more precise: it means from the United States.

Americano can be understood, but many Spanish speakers use it less carefully because América can refer to the whole continent or the Americas, not just the U.S.

So in many Latin American contexts:

• estadounidense = clearer and more neutral

No. Estadounidense has the same form for both masculine and feminine singular.

• Mi hermano es estadounidense.
• Mi hermana es estadounidense.

The plural is:

• estadounidenses

So:

• Mis hermanos son estadounidenses.
• Mis hermanas son estadounidenses.

Because in Spanish, nationalities are normally written with lowercase letters.

So Spanish writes:

• estadounidense
• mexicano
• argentino
• colombiano

This is different from English, where American, Mexican, and so on are capitalized.

Here it is functioning like an adjective of nationality after ser.

• Mi hermano es estadounidense.

This is the normal way to say someone’s nationality.

It can also be used as a noun in some contexts, for example:

• Es un estadounidense.

But for a simple statement of nationality, Spanish usually prefers the version without the article:

• Es estadounidense.

Yes, absolutely.

• Mi hermano es estadounidense.
• Mi hermano es de Estados Unidos.

Both are natural. The first uses a nationality word; the second uses a place of origin phrase.

Very roughly:

• es estadounidense = he is U.S. American / he is American (from the U.S.)
• es de Estados Unidos = he is from the United States

You would say:

• Mi hermana es estadounidense.

Only hermano changes to hermana.

Notice:

• mi stays the same
• es stays the same
• estadounidense stays the same

So the only change is the noun for brother/sister.

A rough pronunciation for English speakers is:

eh-stah-doh-oo-nee-DEN-seh

The stress is on den:

• es-ta-do-u-ni-DEN-se

A few helpful notes:

• Spanish e is usually like eh
• Spanish a is like ah
• the final e is pronounced, not silent
• the d between vowels is softer than English d

Yes, if the context already makes it clear who you mean.

Spanish often leaves things out when they are understood from context.

So all of these can work, depending on the situation:

• Mi hermano es estadounidense.
• Él es estadounidense.
• Es estadounidense.

The full sentence with mi hermano is just more explicit.