ane ha kyuukyuubako no naka no syoudokueki ga sukunaikoto ni kidukimasita.

The sentence breaks down like this:

• 姉は = as for my older sister / my older sister
• 救急箱の中の消毒液が少ないことに = that the disinfectant in the first-aid kit was low / scarce
• 気づきました = noticed

So the overall pattern is:

A は B こと に 気づく
= A notices that B

More literally:

My older sister noticed the fact that the disinfectant in the first-aid kit was low.

は marks the topic of the sentence.

Here, 姉は means something like:

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

It tells you who the sentence is about. The important new information comes later: what she noticed.

If you changed it to 姉が, the nuance would shift more toward identifying who noticed it, as if contrasting her with someone else.

In a sentence like this, 姉 usually means my older sister or the speaker’s older sister, unless context says otherwise.

Japanese often leaves out things like my, your, or the when they are understood from context. So 私の姉 is often shortened to just 姉.

This is a very common Japanese noun-linking pattern.

Step by step:

• 救急箱 = first-aid kit
• 救急箱の中 = the inside of the first-aid kit
• 救急箱の中の消毒液 = the disinfectant in the first-aid kit

So の is linking nouns together:

• AのB = B of A / B in A / A’s B

Here it is not possessive in the English ’s sense so much as a general connector.

Because 消毒液 is the subject of the embedded statement:

消毒液が少ない
= the disinfectant is low / there is little disinfectant

Inside a clause that expresses the content of what was noticed, が is very natural for marking the subject of that clause.

So the sentence has:

• main topic: 姉は
• subject inside the embedded clause: 消毒液が

This is a very common pattern in Japanese: one sentence can contain both a main は topic and an internal が subject.

少ない means few or little, depending on the noun.

With 消毒液 here, it means there was not much disinfectant. In natural English, you might translate it as:

• there was little disinfectant
• the disinfectant was low
• the disinfectant was running low

It refers to quantity, not size.

こと turns the preceding clause into a noun-like unit: the fact that...

So:

• 消毒液が少ない = the disinfectant is low
• 消毒液が少ないこと = the fact that the disinfectant is low

This is necessary because 気づく takes a noun-like object of awareness. You are noticing a fact or a situation.

So:

消毒液が少ないことに気づく
= to notice that the disinfectant is low

Because 気づく commonly takes に.

Pattern:

• X に 気づく = to notice X / to become aware of X

So here:

• 消毒液が少ないことに気づきました = noticed that the disinfectant was low

This is just the standard grammar of the verb 気づく.
It is not usually ことを気づく.

Because the act of noticing happened at a specific point in time.

• 気づきます = notices / will notice
• 気づきました = noticed

Even if the disinfectant is still low now, the sentence is talking about the moment when the older sister became aware of that fact.

That is close grammatically, but it sounds unnatural in English.

A more natural translation would be:

• My older sister noticed that there was little disinfectant in the first-aid kit.
• My older sister noticed that the disinfectant in the first-aid kit was running low.

Japanese often uses structures like Aことに気づく, where a literal English version contains the fact that..., but natural English usually just says noticed that...

Yes. 中 literally means inside or middle, and here it is functioning as a noun.

That is why it can appear in a noun chain:

• 救急箱の中 = the inside of the first-aid kit

You can think of it as:

• inside of the first-aid kit
• the interior of the first-aid kit

救急箱 means first-aid kit or medicine box.

It is made of:

• 救急 = emergency / first aid
• 箱 = box

So 救急箱の中の消毒液 means the disinfectant in the first-aid kit.

In standard grammar here, こと is the natural choice.

You generally say:

• ～ことに気づく = notice that...

Without こと, the clause would not fit as smoothly as the thing being noticed.

So:

• 消毒液が少ないことに気づきました = natural
• omitting こと here would sound incomplete or ungrammatical

Japanese often does not need an equivalent of English there is / there was when the meaning is clear.

消毒液が少ない literally looks like:

• the disinfectant is little

But natural English turns that into:

• there is little disinfectant
• the amount of disinfectant is small
• the disinfectant is low

Japanese often expresses quantity directly with an adjective like 少ない, without needing a separate there is structure.

They are related but not the same.

• 気づく = to notice, to become aware of something
• 分かる = to understand, to know, to realize in the sense of comprehension

In this sentence, the older sister noticed something by observing the situation, so 気づく is the right verb.

If you used 分かった, it would sound more like she came to understand or realized it, rather than specifically noticing it.

Yes, it is in the polite past form because of 気づきました.

The plain form would be:

姉は救急箱の中の消毒液が少ないことに気づいた。

Both mean the same thing, but:

• 気づきました = polite
• 気づいた = plain / casual

The rest of the sentence is neutral standard Japanese.