ane ha bunkasai de gassyou wo happyousuru node, mainiti rensyuusite imasu.

Here, 姉 means older sister, and in normal usage it usually refers to the speaker’s own older sister when talking to someone else.

A useful contrast:

• 姉 = my older sister
• お姉さん = someone else’s older sister / older sister in a more polite form

So in this sentence, the natural reading is my older sister.

は marks 姉 as the topic of the sentence.

So 姉は means something like:

• As for my older sister, ...
• My older sister ...

This tells you that the rest of the sentence is going to say something about her.

If the sentence used が instead, it would sound more like it was identifying or emphasizing who is doing it:

• 姉が... = It is my older sister who... / stronger focus on the subject

In this sentence, は is the natural choice because the speaker is simply talking about their older sister as the topic.

文化祭 means school cultural festival.

The particle で marks the place or event setting where an action happens. So:

• 文化祭で合唱を発表する = perform/present a chorus at the cultural festival

Why not に?

• で = where an action takes place
• に = destination, point in time, or target of existence/arrival in many cases

Since the chorus performance happens at the festival, で is the correct particle.

発表する literally means to announce, to present, or to give a presentation/performance, depending on context.

In this sentence, 合唱を発表する means something like:

• to present a choral performance
• to perform a chorus piece publicly

This may sound a little broader than a simple to sing. The nuance is that the chorus is being presented at an event, not just sung casually.

Compare:

• 合唱する = to sing in chorus
• 合唱を発表する = to give/present a chorus performance

The particle を marks the direct object of the verb.

Here, the thing being presented is 合唱:

• 合唱を発表する = to present a chorus

So 合唱 is the content/performance that is being presented.

ので means because or since.

It connects the reason to the result:

• 文化祭で合唱を発表するので、毎日練習しています。
• Because she will perform a chorus at the cultural festival, she practices every day.

A key nuance: ので often sounds a bit more explanatory, gentle, or neutral than から.

So this sentence feels like a calm explanation of why she is practicing every day.

Both can mean because, but they often feel slightly different:

• から can sound more direct or more personal
• ので often sounds softer and more explanatory

So:

• 発表するから、毎日練習しています。
• 発表するので、毎日練習しています。

Both are grammatical, but ので fits well when giving a reason in a neutral, natural way.

In Japanese, the non-past form covers both:

• present/habitual actions
• future actions

So 発表する can mean:

• presents
• will present / is going to present

In this sentence, because the cultural festival performance has not happened yet, 発表する is understood as a future event:

• will perform/present

This is very normal in Japanese.

This is:

• 練習する = to practice
• 練習して = te-form
• います = part of the ～ています pattern

So 練習しています is 練習する in the ～ている form.

This form often expresses:

1. an action in progress
   
   • is practicing
2. a repeated or habitual action
   
   • practices regularly / has been practicing

Because the sentence includes 毎日, the meaning here is best understood as a repeated current habit:

• she is practicing every day
• she has been practicing every day

It does not mean she is literally practicing at this exact second.

毎日 is an adverbial time expression, and expressions like this often appear without any particle.

So:

• 毎日練習しています = practices every day

This is completely natural.

Many common time words behave this way:

• 毎日 = every day
• 今日 = today
• 明日 = tomorrow
• 去年 = last year

Sometimes time expressions do take particles, but 毎日 very often appears with no particle at all.

Yes. The subject is omitted, but it is understood to be the same person introduced at the start: 姉.

So the full idea is:

• My older sister will perform a chorus at the cultural festival, so she is practicing every day.

Japanese often omits subjects when they are already clear from context. Once 姉は sets the topic, the rest of the sentence naturally continues to describe her unless something changes.

Yes. Japanese word order is fairly flexible as long as the relationships are clear.

For example, these are both natural:

• 姉は文化祭で合唱を発表するので、毎日練習しています。
• 姉は毎日、文化祭で合唱を発表するので練習しています。

But the original version is very natural because 毎日 is placed right before the verb phrase it modifies:

• every day practices

That placement makes it easy to understand.

The comma 、 helps separate the reason clause from the main clause:

• 文化祭で合唱を発表するので、
• 毎日練習しています。

It makes the sentence easier to read.

It is especially common to put a comma after a long reason clause like this, though punctuation can vary somewhat by style.

No. Standard Japanese is normally written without spaces between words.

This sentence would usually be written as:

• 姉は文化祭で合唱を発表するので、毎日練習しています。

The spaces in your version are just for learners, to make the word boundaries easier to see.