Mi hermana tiene una ceja un poco más alta que la otra.

Because Spanish often uses tener for physical features someone has.

• Mi hermana tiene una ceja... = My sister has one eyebrow...
• This is the normal way to describe body parts and physical characteristics in many contexts.

Using es would sound wrong here, because ser links a subject to a quality, but the sentence is built around possession of a body part:

• Tiene los ojos azules
• Tiene el pelo corto
• Tiene una ceja más alta que la otra

You could also describe the eyebrow itself with estar or ser in a different structure, for example:

• Una de sus cejas está más alta que la otra But in your sentence, tiene is the natural choice.

Because mi without an accent means my.

• mi hermana = my sister

With an accent, mí means me after a preposition:

• para mí = for me
• sin mí = without me

So here it must be mi, because it is a possessive adjective.

Because ceja is a feminine noun in Spanish: la ceja.

Words that describe or refer back to ceja must agree with it in gender and number:

• una ceja
• más alta
• la otra

This is standard agreement in Spanish:

• noun: ceja
• adjective: alta
• determiner/pronoun: la otra

If the noun were masculine, those words would change:

• un ojo más alto que el otro would be grammatically masculine, though not a very natural example semantically.

Spanish is comparing the two eyebrows one by one:

• una ceja = one eyebrow
• la otra = the other one

This is a very natural way to compare two things in Spanish and in English.

It avoids repeating ceja the second time. A fuller version would be:

• Mi hermana tiene una ceja un poco más alta que la otra ceja

But that sounds repetitive, so Spanish normally says just la otra.

This is the standard Spanish comparative pattern:

• más + adjective + que = more + adjective + than

So:

• más alta que = higher than

Examples:

• más grande que = bigger than
• más rápido que = faster than
• más interesante que = more interesting than

In your sentence:

• una ceja un poco más alta que la otra = one eyebrow a little higher than the other

Because que is used for comparisons:

• más alta que la otra = higher than the other one

Spanish uses de after más mainly with numbers or quantities:

• más de dos horas = more than two hours
• más de 20 euros = more than 20 euros

So:

• comparison of two things -> más ... que
• amount/number -> más de

Un poco means a little or slightly.

It softens the comparison:

• más alta = higher
• un poco más alta = a little higher / slightly higher

So the sentence does not say the difference is big. It suggests only a small difference.

Very similar alternatives are:

• un poquito más alta = a tiny bit higher / more informal
• ligeramente más alta = slightly higher / a bit more formal

Because un poco modifies the comparative phrase más alta.

Structure:

• un poco + más + adjective

So:

• un poco más alta
• un poco más grande
• un poco más caro

This is the normal word order in Spanish.

You would not usually say:

• más un poco alta ❌

Yes. That is perfectly correct.

• Mi hermana tiene una ceja más alta que la otra.
• Mi hermana tiene una ceja un poco más alta que la otra.

The difference is only nuance:

• without un poco: a neutral comparison
• with un poco: the difference is small

So un poco adds subtlety, not a grammatical necessity.

Not naturally in this sentence.

• la otra = the other one
• otra = another / other

Here we are talking about a specific pair: her two eyebrows. So Spanish uses la otra to mean the remaining one of the two.

Compare:

• una ceja más alta que la otra = one eyebrow higher than the other one
• una ceja más alta que otra sounds incomplete or unnatural here

So la otra is the right choice.

Alto/alta can mean tall, but it can also mean high depending on context.

Here it means higher, not taller:

• una ceja más alta = an eyebrow that sits higher

Spanish uses alto/alta for vertical position too:

• una pared alta = a tall wall
• una montaña alta = a high mountain
• la ceja está más alta = the eyebrow is higher

So the exact English translation depends on context.

Yes, but it would be a different structure.

For example:

• Mi hermana tiene las cejas desiguales. = My sister’s eyebrows are uneven.
• Mi hermana tiene una ceja más alta que la otra. = My sister has one eyebrow higher than the other.

Your original sentence is more specific and more visual. It directly compares one eyebrow with the other.

So both are possible, but the original is more precise.

In most of Spain, ceja is pronounced approximately THE-kha.

More exactly:

• ce -> like English th in think
• j -> a strong throaty sound, like the ch in Scottish loch or German Bach

So:

• ceja ≈ THE-kha

And in the full sentence:

• Mi hermana tiene una ceja un poco más alta que la otra would sound, in broad approximation:
• Mi er-MA-na TYE-ne OO-na THE-kha oom PO-co mas AL-ta ke la O-tra

If you are learning Spain Spanish, that ce sound as th is important in most regions of Spain.

It sounds natural.

A Spanish speaker could definitely say:

• Mi hermana tiene una ceja un poco más alta que la otra.

It is a normal, idiomatic way to describe uneven eyebrows or a slight asymmetry in the face.

It sounds more natural than a very literal structure like:

• Una de las cejas de mi hermana está un poco más alta que la otra

That version is also correct, but your original sentence is simple and very natural in everyday speech.