Mi sobrina tiene el pelo rubio y rizado.

In Spanish, when talking about body parts, it is very common to use the definite article (el / la / los / las) instead of a possessive like su.

So Spanish prefers:

• Mi sobrina tiene el pelo rubio y rizado.

rather than:

• Mi sobrina tiene su pelo rubio y rizado.

Using su pelo is not impossible, but it often sounds unnecessary here. Since it is already clear the hair belongs to my niece, Spanish naturally uses el pelo.

This is a very common pattern:

• Me duele la cabeza = My head hurts
• Tiene los ojos azules = He/She has blue eyes

Because rubio and rizado are describing el pelo, not mi sobrina.

• pelo is masculine singular
• so the adjectives must also be masculine singular:
  • rubio
  • rizado

If the adjectives described the niece herself, then they would be feminine:

• Mi sobrina es rubia y rizada.

That sentence is also possible, but it focuses more on what she is like physically, while tiene el pelo rubio y rizado focuses specifically on her hair.

Yes, you can.

Both are natural, but they are not exactly identical in emphasis:

• Mi sobrina tiene el pelo rubio y rizado
  = specifically talks about her hair

• Mi sobrina es rubia y rizada
  = describes her overall appearance as a blonde, curly-haired girl/woman

So the original sentence is a bit more explicit and literal about the hair itself.

In Spanish, pelo is commonly used as a singular collective noun when referring to someone’s hair in general.

So:

• tiene el pelo rubio = she has blonde hair

Spanish normally does not say los pelos for this meaning.
Los pelos usually means individual hairs or can sound odd in this context.

So for someone’s hairstyle or hair colour, el pelo is the normal choice.

Both can mean hair, but there is a slight difference in tone:

• pelo = the everyday, most common word
• cabello = a bit more formal or literary

In normal conversation, especially in Spain, pelo is very common and completely natural here:

• Tiene el pelo rubio y rizado.

You may also hear:

• Tiene el cabello rubio y rizado.

That is correct too, just slightly more formal in tone.

In Spanish, adjectives usually come after the noun they describe.

So:

• el pelo rubio
• la casa blanca
• los ojos verdes

That is the normal order.

Sometimes Spanish puts adjectives before the noun, but that usually changes the style or emphasis. In this sentence, the standard order is definitely:

• el pelo rubio y rizado

Because Spanish, like English, often joins the last two adjectives with y (and).

So:

• rubio y rizado = blonde and curly

A comma is not needed with just two adjectives. If there were more, you might write:

• Tiene el pelo largo, rubio y rizado.

That means long, blonde, and curly hair.

In this kind of sentence, rubio usually means blonde / fair-haired.

So:

• pelo rubio = blonde hair

In Spanish, rubio/rubia can describe a person or their hair. It generally refers to light-coloured hair, from blonde to fair/light hair.

For a beginner, the safest translation is simply:

• rubio = blonde

Usually rizado means curly in appearance. It does not always tell you whether the hair is naturally curly or styled that way.

So:

• pelo rizado = curly hair

If Spanish wants to be more specific, it can add more information, but on its own rizado simply describes the texture/look.

Spanish normally uses the short possessive before the noun:

• mi sobrina = my niece

This is the most common and neutral way to say it.

The longer form with mío / mía / míos / mías usually comes after the noun and is more emphatic or stylistic:

• una sobrina mía = a niece of mine
• la sobrina mía can exist in some contexts, but it is much less common and not the normal choice here

So mi sobrina is exactly what you would expect.

Yes, if the subject is already clear from context.

Spanish often omits subject pronouns, and it can also omit a noun phrase if everyone already knows who is being talked about.

So if you are already discussing your niece, you could simply say:

• Tiene el pelo rubio y rizado.

That means:

• She has blonde, curly hair.

But if you are introducing the person, Mi sobrina is useful and natural.

Yes. Tener is very commonly used in Spanish to describe physical features.

So Spanish often says:

• tener el pelo... = to have hair that is...
• tener los ojos... = to have eyes that are...
• tener barba = to have a beard

This is one of the most basic and useful uses of tener.

In this sentence:

• tiene = she has

because it is the third person singular form of tener.

Yes, that is grammatically correct.

The order of the adjectives can sometimes change without changing the basic meaning:

• rubio y rizado
• rizado y rubio

Both work.

That said, rubio y rizado may sound slightly more natural to many speakers here, but the difference is small. The sentence still means the same thing: her hair is blonde and curly.