Моя сестра любит носить длинную косу, а на праздник обычно делает мягкие кудри.

Because моя has to agree with сестра in gender, number, and case.

• сестра is feminine
• singular
• nominative case here, because it is the subject

So the possessive pronoun must also be feminine singular nominative:

• мой = masculine
• моя = feminine
• моё = neuter
• мои = plural

So моя сестра means my sister.

This is a very common Russian pattern:

• любить + infinitive = to like/love doing something

So:

• любит носить = likes to wear
• literally, loves to wear

Here, любит is the 3rd person singular form of любить, because the subject is моя сестра.

Compare:

• Я люблю читать. = I like reading.
• Он любит готовить. = He likes cooking.
• Моя сестра любит носить длинную косу. = My sister likes to wear a long braid.

Russian makes a distinction that English often does not.

• носить = to wear, to have on, or more generally to go around with
• надевать = to put on something

So in this sentence, the idea is that she wears her hair in that style, not that she is in the act of putting it on.

With hairstyles, носить is very natural:

• носить косу = to wear one’s hair in a braid
• носить короткие волосы = to wear short hair
• носить бороду = to wear a beard

So любит носить длинную косу means she likes having/wearing her hair in a long braid.

Because косу is the direct object of носить, so it goes into the accusative case.

The adjective has to agree with the noun, so both words change:

• nominative: длинная коса
• accusative: длинную косу

This is a very typical feminine singular pattern:

• новая книга → новую книгу
• красивая сумка → красивую сумку
• длинная коса → длинную косу

So the sentence uses the accusative because she wears the braid.

Here коса means a braid or plait of hair.

So длинная коса is a long braid.

This word can have other meanings in different contexts, so context matters. In this sentence, because we are talking about hair and then about curls, it very clearly means a hairstyle.

А often links two parts of a sentence while also showing a contrast or change of focus.

Here the idea is something like:

• she usually wears one hairstyle,
• but / while / and on the other hand, for a special occasion she does another one.

So а works very naturally because the sentence contrasts her everyday preference with what she does for a celebration.

Very roughly:

• и = and
• а = and/but, with contrast or comparison
• но = but, often stronger contradiction

In this sentence, а is the best choice because it is a soft contrast, not a sharp contradiction.

На праздник means for a holiday / for a celebration / for a special occasion.

This is an idiomatic Russian way to express doing something for an event.

So:

• на праздник = for the celebration
• на свадьбу = for the wedding
• на день рождения = for the birthday

It does not literally mean physical location here. It is more about the occasion.

A learner might expect something like в праздник, but that usually means on a holiday / during a holiday in a more general time sense.
Here на праздник sounds more like preparing a look for the event.

Обычно means usually, and Russian word order is flexible.

In this sentence, обычно делает is a very natural order:

• на праздник обычно делает мягкие кудри

This places обычно close to the verb and sounds neutral.

You could move обычно in some contexts, but the nuance changes slightly:

• Моя сестра обычно носит длинную косу... = usually, in general
• ...а на праздник обычно делает мягкие кудри. = when it is a celebration, she usually does soft curls

So the placement helps connect usually specifically with what she does for a celebration.

In Russian, делать is very commonly used for creating or styling a look, especially with hair.

So:

• делать кудри = to do/make curls
• делать причёску = to do one’s hair / make a hairstyle
• делать макияж = to do makeup

This may sound a little different from English, where we often say curl her hair or wear her hair in curls, but in Russian делать is very normal here.

So делает мягкие кудри means she styles her hair into soft curls.

This is also the accusative case, because кудри is the direct object of делает.

In this phrase:

• nominative plural: мягкие кудри
• accusative plural: мягкие кудри

For inanimate plural nouns, the nominative and accusative are often the same. That is why the form does not visibly change here.

So even though the form looks like nominative, its function in the sentence is accusative.

Because curls are naturally thought of as multiple strands or locks of hair.

Russian usually uses the plural for this idea:

• кудри = curls
• мягкие кудри = soft curls

Using the singular would sound unusual here. Just as in English we usually say curls, Russian normally uses the plural too.

Russian often omits subject pronouns when the subject is already clear from context.

In this sentence, the subject is introduced at the start:

• Моя сестра...

After that, it is understood that the second verb still refers to her:

• любит носить...
• делает...

So Russian does not need она here.

If you added она, it would still be grammatical, but usually less natural unless you want extra emphasis:

• Моя сестра любит носить длинную косу, а на праздник она обычно делает мягкие кудри.

That version puts a bit more focus on she.

No, Russian word order is fairly flexible, but the sentence as given is neutral and natural.

The original order:

• Моя сестра любит носить длинную косу, а на праздник обычно делает мягкие кудри.

This sounds smooth and standard.

You could rearrange parts for emphasis, for example:

• На праздник моя сестра обычно делает мягкие кудри.
• Длинную косу моя сестра любит носить...

But changing the order changes what gets highlighted. The original version presents the information in a very natural, textbook-friendly way.

Yes, and that is completely normal in Russian.

They focus on two different aspects:

• носить длинную косу = to wear one’s hair in a long braid
• делать мягкие кудри = to style one’s hair into soft curls

So one expression describes the hairstyle as something she has/wears, and the other describes the action of creating the hairstyle.

That combination sounds very natural to a Russian speaker.