Η αδερφή μου είναι μελαχρινή και έχει ίσια μαλλιά, ενώ εγώ είμαι ξανθός και έχω σγουρά.

In Greek, possessives like μου (my) usually come after the noun, not before it.

So:

• η αδερφή μου = my sister
• literally: the sister my

This is completely normal in Greek. Very often, Greek also keeps the definite article:

• ο αδερφός μου = my brother
• η μητέρα μου = my mother
• το σπίτι μου = my house

So even though English says my sister, Greek commonly says the sister my.

Μου means my here.

It is an unstressed possessive form and is used for:

• my
• also, in other contexts, to me / me depending on the sentence structure

In this sentence, it is possessive:

• η αδερφή μου = my sister

Unlike English my, Greek μου does not change for gender or number of the thing owned:

• ο φίλος μου = my friend
• η φίλη μου = my friend
• τα βιβλία μου = my books

So μου stays the same.

Because adjectives in Greek must agree with the gender and number of the person or thing they describe.

Here:

• η αδερφή μου is feminine singular, so the adjective is feminine singular: μελαχρινή
• εγώ refers to a male speaker in this sentence, so the adjective is masculine singular: ξανθός

So:

• μελαχρινός = dark-haired/dark-featured, masculine
• μελαχρινή = feminine
• ξανθός = blond, masculine
• ξανθιά = feminine

If the speaker were female, you would expect:

• ενώ εγώ είμαι ξανθιά

Because they are predicate adjectives, used after the verb to be.

Greek works like English here:

• είναι μελαχρινή = is dark-haired
• είμαι ξανθός = I am blond

So the structure is:

• subject + είμαι/είναι + adjective

Examples:

• Είμαι κουρασμένος. = I am tired.
• Είναι ψηλή. = She is tall.

They are different verbs:

• είναι = is
• έχει = has
• έχω = I have

So:

• Η αδερφή μου είναι μελαχρινή = My sister is dark-haired
• και έχει ίσια μαλλιά = and she has straight hair
• εγώ είμαι ξανθός = I am blond
• και έχω σγουρά = and I have curly hair

Greek uses to be for some descriptions and to have for others, just as English does.

Because the verb έχω (to have) changes depending on the subject.

Here are the relevant forms:

• εγώ έχω = I have
• αυτός/αυτή/αυτό έχει = he/she/it has

So:

• η αδερφή μου έχει = my sister has
• εγώ έχω = I have

This is basic verb conjugation in Greek.

In Greek, μαλλιά is very commonly used in the plural when talking about someone’s hair in general.

So:

• ίσια μαλλιά = straight hair
• σγουρά μαλλιά = curly hair

This is one of those places where Greek and English organize the idea differently. English usually treats hair as a mass noun, but Greek often uses the plural μαλλιά.

You may also see μαλλί in some contexts, but for hairstyle or hair on the head in everyday Greek, μαλλιά is very common.

Because Greek can omit the noun when it is obvious from context.

The full second part would be:

• έχω σγουρά μαλλιά = I have curly hair

But since μαλλιά was already mentioned earlier, Greek naturally leaves it out:

• έχω σγουρά = I have curly ones / I have curly hair

This works because σγουρά is in the same form that would agree with μαλλιά.

So the omitted noun is understood.

Because they agree with μαλλιά, which is neuter plural.

So the adjectives also appear in the neuter plural form:

• ίσιος / ίσια / ίσιο = straight
• σγουρός / σγουρή / σγουρό = curly

With μαλλιά:

• ίσια μαλλιά
• σγουρά μαλλιά

Even when μαλλιά is omitted, the adjective stays in the form that matches it:

• έχω σγουρά = I have curly hair
  literally, I have curly [ones]

Ενώ here means while, whereas, or but/while by contrast.

It connects the two parts of the sentence and shows contrast:

• My sister is dark-haired and has straight hair, whereas I am blond and have curly hair.

So it is not just a simple and. It introduces a comparison or contrast between the sister and the speaker.

That’s true: Greek often omits subject pronouns because the verb ending already shows who the subject is.

For example:

• έχω already means I have
• είμαι already means I am

So Greek could theoretically say:

• ενώ είμαι ξανθός και έχω σγουρά

But εγώ is included for emphasis or contrast:

• while I am blond and have curly hair

Since the sentence is comparing my sister with me, using εγώ sounds natural.

A learner may also encounter αδελφή.

Both exist, but:

• αδερφή is very common in everyday Modern Greek
• αδελφή is more formal or traditional

Similarly:

• αδερφός / αδελφός = brother

So in ordinary spoken Greek, αδερφή μου sounds very natural.

Yes, absolutely.

Both are correct:

• ...και έχω σγουρά.
• ...και έχω σγουρά μαλλιά.

The shorter version avoids repeating μαλλιά, because it is already understood. The longer version is more explicit.

Greek often prefers the shorter version when the meaning is clear.

The main change would be the adjective describing the speaker:

• ενώ εγώ είμαι ξανθιά και έχω σγουρά

Why?

Because ξανθός is masculine, while ξανθιά is feminine.

Notice that έχω σγουρά does not change, because σγουρά refers to μαλλιά, and μαλλιά is still neuter plural.

So:

• male speaker: είμαι ξανθός
• female speaker: είμαι ξανθιά

But in both cases:

• έχω σγουρά (μαλλιά)