Minha sobrinha não gosta de ficar sozinha em casa na sexta-feira.

Because gostar normally takes the preposition de before a noun or an infinitive.

• gostar de + noun → Ela gosta de café.
• gostar de + infinitive → Ela gosta de ficar em casa.

So in your sentence, não gosta de ficar sozinha means doesn’t like being/staying alone.

This is a very common pattern in Portuguese, and English speakers often forget the de.

Here, ficar means to stay or to be in a temporary situation.

So ficar sozinha em casa means something like:

• to stay home alone
• to be alone at home

Portuguese ficar is very flexible. It can mean:

• to stay/remain → Vou ficar em casa.
• to become → Ela ficou triste.
• in some contexts, even to hook up/date casually

In this sentence, the meaning is clearly to stay/be.

Because sozinha agrees with minha sobrinha, which is feminine singular.

• sozinho = masculine singular
• sozinha = feminine singular
• sozinhos = masculine plural
• sozinhas = feminine plural

Since sobrinha is feminine, the adjective must also be feminine:

• Minha sobrinha ... sozinha
• Meu sobrinho ... sozinho

Em casa is a very common fixed expression meaning at home or home.

So:

• Estou em casa. = I’m at home.
• Ela ficou em casa. = She stayed home.

You usually do not say na casa when you mean at home in a general sense.

But na casa is used when you mean a specific house:

• Estou na casa da minha tia. = I’m at my aunt’s house.

So:

• em casa = at home
• na casa de alguém = at someone’s house

Na is a contraction of em + a.

• em = in/on/at
• a = the

So:

• na sexta-feira literally comes from em a sexta-feira

In natural English, this usually means on Friday.

Days of the week in Portuguese often appear with the article:

• na segunda-feira
• na terça-feira
• na quarta-feira
• na quinta-feira
• na sexta-feira

In this sentence, it most naturally means on Friday, referring to a typical situation or a specific Friday depending on context.

Portuguese can be a little flexible here. To make it clearly habitual, people often say:

• às sextas-feiras = on Fridays

So compare:

• Minha sobrinha não gosta de ficar sozinha em casa na sexta-feira.
  = My niece doesn’t like staying home alone on Friday / on a Friday.
• Minha sobrinha não gosta de ficar sozinha em casa às sextas-feiras.
  = My niece doesn’t like staying home alone on Fridays.

In Portuguese, the normal way to negate a verb is to put não directly before it.

• Ela gosta. = She likes.
• Ela não gosta. = She doesn’t like.

So:

• Minha sobrinha não gosta...

This is much simpler than English, because Portuguese does not need a helper like do/does.

Minha sobrinha is the normal and neutral word order for my niece.

In Portuguese, possessives usually come before the noun:

• meu irmão
• minha mãe
• minha sobrinha

You can sometimes put the possessive after the noun in special or literary contexts, but it is not the usual everyday pattern here.

Also notice the agreement:

• meu for masculine singular nouns
• minha for feminine singular nouns

Since sobrinha is feminine, it must be minha sobrinha.

Yes. In Brazilian Portuguese, both are possible:

• Minha sobrinha não gosta...
• A minha sobrinha não gosta...

The version without the article is very common and natural in Brazil. The version with the article is also possible, depending on region, style, and emphasis.

For a learner, the important thing is:

• both can be correct
• dropping the article before possessives is very common in Brazilian Portuguese

The de does not connect to sozinha. It belongs to the verb gostar.

The structure is:

• [Minha sobrinha] [não gosta] [de ficar sozinha em casa] [na sexta-feira].

So de ficar sozinha em casa is the whole thing she does not like.

Inside that part:

• ficar = to stay/be
• sozinha = alone
• em casa = at home

So the sentence is built like:

• subject: Minha sobrinha
• verb: não gosta de
• complement: ficar sozinha em casa
• time expression: na sexta-feira

Because sozinha is describing the state she is in while staying at home.

After verbs like ficar, adjectives often come after the verb:

• ficar sozinho
• ficar triste
• ficar calado

So:

• ficar sozinha = to be/stay alone

Putting sozinha before ficar would not sound natural in this sentence.

Yes. The weekday names that use -feira are normally written with a hyphen:

• segunda-feira
• terça-feira
• quarta-feira
• quinta-feira
• sexta-feira

But:

• sábado
• domingo

do not use -feira.

A helpful approximate pronunciation is:

MEE-nya soh-BREEN-ya now GOH-sta jeh fee-KAR soh-ZEEN-ya eyn KAH-za na SEKS-ta FEY-ra

A few pronunciation notes:

• minha often sounds close to mi-nya
• não has a nasal sound, something like now̃
• de in Brazilian Portuguese often sounds like jee/djee before some sounds in connected speech, though learners can safely say a simple deh
• sozinha has the nh sound like Spanish ñ or English ny in canyon
• sexta-feira has stress on SEX and FEI

Yes, that is a very natural translation.

Depending on context, you could also translate it as:

• My niece doesn’t like being home alone on Friday.
• My niece doesn’t like staying at home by herself on Friday.

All of those match the Portuguese well. The exact English choice depends on what style of English you want, but the Portuguese structure stays the same.