Η αδερφή μου λέει ότι το ασπρόμαυρο της πάει περισσότερο, ενώ εγώ προτιμώ ένα μονόχρωμο στυλ.

In Greek, family words and many other nouns normally keep the definite article even when there is a possessive like μου.

So:

• η αδερφή μου = my sister
• literally, it looks like the sister of mine

This is completely normal Greek structure. English drops the, but Greek usually does not.

Yes. Both mean sister.

• αδελφή is the more traditional / formal spelling
• αδερφή is very common in everyday modern Greek

A learner will hear and see both. In normal conversation, αδερφή is extremely natural.

μου means my here, but grammatically it is a weak possessive pronoun, not a separate adjective like English my.

So:

• ο αδερφός μου = my brother
• η αδερφή μου = my sister
• το σπίτι μου = my house

It comes after the noun, unlike English.

ότι means that and introduces a clause:

• λέει ότι... = she says that...

So the structure is:

• Η αδερφή μου λέει = My sister says
• ότι το ασπρόμαυρο της πάει περισσότερο = that black-and-white suits her more

In spoken Greek, πως can also be used in many cases with a similar meaning:

• λέει πως...
• λέει ότι...

Both are common.

That is a very common Greek expression.

• πάει literally comes from πάω = to go
• but κάτι πάει σε κάποιον means something suits someone

So:

• της πάει = it suits her
• literally: it goes to her

Other examples:

• Το κόκκινο σου πάει. = Red suits you.
• Αυτό το φόρεμα της πάει πολύ. = This dress suits her a lot.

Here της means to her.

Here ασπρόμαυρο is being used as a neuter noun-like word, meaning something like the black-and-white look / black-and-white style / black-and-white color scheme.

Greek often uses the neuter singular of an adjective to talk about a general concept, style, or quality.

So:

• το ασπρόμαυρο = black-and-white as a style/look
• not necessarily one specific object

This is why the neuter article το is used.

περισσότερο means more.

In this sentence, it means something like:

• it suits her more
• it suits her better
• she looks better in black-and-white

It implies a comparison, even if the second thing is not stated explicitly. The unstated idea is more than other styles/colors.

ενώ can mean while, but very often it is used to show contrast, like:

• while
• whereas
• but

In this sentence, it is contrastive:

• Η αδερφή μου... ενώ εγώ...
• My sister..., whereas I...

So it is not mainly about two actions happening at the same time. It is contrasting two preferences.

Yes, Greek could leave it out.

Greek often omits subject pronouns because the verb ending already shows the subject. So:

• προτιμώ already means I prefer

But εγώ is added here for emphasis and contrast:

• while I prefer...
• as for me, I prefer...

This works especially well after ενώ, because the sentence is comparing my sister and me.

They are different ideas:

• ασπρόμαυρο = black-and-white
• μονόχρωμο = monochrome / single-color / plain-colored

So the contrast is not between two identical things. The speaker’s sister prefers a black-and-white look, while the speaker prefers a single-color / monochrome style.

Depending on context, μονόχρωμο can suggest a cleaner, simpler, less mixed visual style.

This is just a normal noun phrase:

• ένα = a
• μονόχρωμο = monochrome / single-color
• στυλ = style

So:

• ένα μονόχρωμο στυλ = a monochrome style

Both ένα and μονόχρωμο are neuter singular because στυλ is treated as a neuter noun in Greek.

The word order is natural, but Greek is more flexible than English.

This order puts the focus nicely on το ασπρόμαυρο as the thing being discussed:

• το ασπρόμαυρο της πάει περισσότερο

You could also hear similar variations depending on emphasis, but this version sounds very natural and straightforward. Greek often moves words around for focus more easily than English does.

It functions like an indirect object clitic, meaning to her.

So:

• μου = to me / my
• σου = to you / your
• της = to her / her
• του = to him / his

In της πάει, it means:

• it goes to her
• therefore, it suits her

So although English uses her, the Greek idea is closer to to her.