Mi sobrina tiene once años, y su hermano tiene trece.

In Spanish, age is expressed with tener (to have), not ser or estar (to be).

So:

• Mi sobrina tiene once años = literally My niece has eleven years
• Su hermano tiene trece = literally Her brother has thirteen

This is just the normal Spanish pattern:

• Tengo veinte años = I am twenty years old
• ¿Cuántos años tienes? = How old are you?

Using ser here would sound wrong in Spanish.

Because once means eleven, and anything other than one takes the plural noun años.

So:

• un año = one year
• once años = eleven years
• trece años = thirteen years

Spanish matches English here in meaning: one year, but eleven years.

Because años is understood from the first part, so Spanish can leave it out to avoid repetition.

Full version:

• Mi sobrina tiene once años, y su hermano tiene trece años.

More natural, less repetitive version:

• Mi sobrina tiene once años, y su hermano tiene trece.

Both are correct. The shorter version sounds very natural because the context already makes it clear that trece means thirteen years old.

Su is a possessive adjective meaning his, her, your (formal singular), their, or your (plural, in Spain mostly su is formal/plural in some contexts but vuestro is also used).

In this sentence, because of the meaning and context, su hermano means her brother.

So:

• mi sobrina = my niece
• su hermano = her brother

Spanish su is less specific than English his/her/their, so context usually tells you whose brother it is.

Because the speaker is talking about:

• my niece = mi sobrina
• her brother = su hermano

The possessives agree with the owner, not with the thing owned.

So:

• mi = my
• su = his/her/their/your formal

And the nouns are:

• sobrina = niece
• hermano = brother

Yes. In Spanish, many nouns referring to people change form depending on gender.

Here:

• sobrina = niece
• sobrino = nephew
• hermano = brother
• hermana = sister

So the sentence is specifically talking about:

• a female niece: sobrina
• a male brother: hermano

This is a very common pattern in Spanish, though not every noun follows it perfectly.

Because Spanish often leaves out subject pronouns when they are not needed.

Instead of saying:

• Mi sobrina ella tiene once años

Spanish normally says:

• Mi sobrina tiene once años

That is enough, because the subject is already clear.

Spanish does this all the time:

• Tengo hambre = I am hungry
• Vivimos en Madrid = We live in Madrid

Adding the pronoun is only done when you want emphasis, contrast, or clarity.

No. Once and trece stay the same.

So you can say:

• once años
• once libros
• trece hermanas
• trece coches

These numbers do not change form for gender.

A number like uno is different, because it can change:

• un hermano
• una hermana

But once and trece do not.

In standard Spain Spanish:

• once is pronounced roughly ON-theh
• trece is pronounced roughly TREH-theh

That is because in much of Spain, the letter c before e or i sounds like the th in think.

So:

• ce sounds like theh
• ci sounds like thee

In many Latin American accents, those same words sound more like:

• ON-se
• TRE-se

Both are correct in their own varieties of Spanish.

Usually, no. In standard Spanish, you normally do not put a comma before y when joining two closely related parts of a sentence.

So the more typical version would be:

• Mi sobrina tiene once años y su hermano tiene trece.

The version with the comma is understandable, but in most ordinary writing, Spanish would leave it out.

Yes, absolutely.

These both work:

• Su hermano tiene trece años
• Su hermano tiene trece

The second one is shorter because años is already understood.

If you want to be fully explicit, especially for a beginner, adding años can be helpful.

Because ñ is its own letter in Spanish, different from n.

• año = year
• ano = a completely different word

So the ñ is very important. It is not just a decoration or accent mark. It changes the word itself.

This is something English speakers need to watch carefully, because leaving out the tilde over ñ can create a different meaning.

Yes. By itself, su hermano could mean:

• his brother
• her brother
• your brother (formal)
• their brother

Spanish relies on context more than English here.

In this sentence, the meaning makes it clear that it refers to her brother, meaning the niece’s brother. If there were any ambiguity, Spanish could make it clearer with:

• el hermano de ella = her brother
• el hermano de él = his brother

But in normal speech, su hermano is usually enough.