Η αδερφή μου βασίζεται στον χάρτη του κινητού της γιατί δεν ξέρει καλά την πόλη.

Yes. Αδερφή is a very common Modern Greek form and sounds natural in everyday speech.

You may also see αδελφή, which is the older and more formal form. In modern everyday Greek:

• αδερφή = very common, natural
• αδελφή = also correct, often a bit more formal or traditional

So in this sentence, Η αδερφή μου is completely normal for my sister.

In Greek, short possessive words like μου (my), σου (your), της (her) usually come after the noun:

• η αδερφή μου = my sister
• το σπίτι μου = my house

This is the standard pattern in everyday Greek.

Putting the possessive before the noun is much less common and usually sounds emphatic, poetic, or more marked. So η αδερφή μου is the normal way to say it.

Greek often leaves out subject pronouns when they are not needed. This is very common.

The verb form βασίζεται already tells you the subject is:

• third person
• singular

So Greek does not need to say she explicitly.

If you added αυτή, it would usually sound more emphatic:

• Η αδερφή μου βασίζεται... = My sister relies...
• Αυτή βασίζεται... = She relies... / She is the one who relies...

Here βασίζεται means relies on or depends on.

It comes from βασίζομαι, which is one of those Greek verbs that has middle/passive-looking endings but an active meaning in English.

So:

• βασίζομαι σε κάτι = I rely on something
• βασίζεται σε κάτι = he/she/it relies on something

In this sentence:

• Η αδερφή μου βασίζεται... = My sister relies...

So even though it looks a bit like a passive form, here it is simply the normal way to say she relies.

Στον is a contraction of:

• σε
  • τον → στον

Here:

• σε = on / to / at (depending on context)
• τον = masculine singular article in the accusative

So:

• στον χάρτη = on the map

This is very common in Greek:

• σε + το → στο
• σε + την → στη(ν)
• σε + τον → στον

Because after σε, Greek uses the accusative case.

The noun is:

• nominative: ο χάρτης = the map
• accusative: τον χάρτη = the map

Since the phrase is σε τον χάρτη → στον χάρτη, the noun has to be in the accusative:

• χάρτη

So the ending changes because of grammar, not because the meaning changes.

Literally, it means:

• του κινητού = of the mobile phone
• της = her

So the whole phrase is literally:

• the map of her mobile phone

In more natural English, that usually means:

• the map on her phone
• her phone’s map
• the map app on her phone

Greek often uses this kind of genitive structure where English might prefer a different phrasing.

Here της means her, and it refers to η αδερφή μου.

So:

• του κινητού της = of her phone

The full idea is:

• My sister relies on the map on her phone...

Even though της comes later in the sentence, Greek speakers naturally understand that it refers back to my sister.

Yes, γιατί here means because.

So:

• ...γιατί δεν ξέρει καλά την πόλη = ...because she doesn’t know the city well

You could also use επειδή, which also means because.

Very roughly:

• γιατί = very common, everyday, natural
• επειδή = also common, sometimes a bit more formal or slightly more explicit

In this sentence, γιατί sounds perfectly natural.

This is the natural Greek way to say she doesn’t know the city well.

Breakdown:

• δεν = not
• ξέρει = knows
• καλά = well
• την πόλη = the city

So literally:

• she doesn’t know well the city

But natural English is:

• she doesn’t know the city well

A useful point is that Greek often uses ξέρω for being familiar with a place:

• Ξέρω την πόλη = I know the city
• Δεν ξέρω καλά την πόλη = I don’t know the city well

This is very idiomatic Greek.

Greek uses the definite article much more often than English.

Here την πόλη means the city, referring to a specific city that is understood from the situation.

So the sentence means she does not know that city / the city she is in / the city being discussed very well.

If you said μια πόλη, that would mean a city, which would change the meaning.

Yes. Greek word order is fairly flexible.

The original sentence is:

• Η αδερφή μου βασίζεται στον χάρτη του κινητού της γιατί δεν ξέρει καλά την πόλη.

A different but natural version would be:

• Η αδερφή μου δεν ξέρει καλά την πόλη, γι’ αυτό βασίζεται στον χάρτη του κινητού της.
• My sister doesn’t know the city well, so she relies on the map on her phone.

The original version puts the main action first (she relies on the map) and then gives the reason (because she doesn’t know the city well).

So yes, the order can change, but the original is completely natural.