ani wa senmenjo ni sekken ga naikoto ni kizuite, kaeri ni katte kite kureta.

兄 is the plain word for my older brother when talking about him to someone else.

A very common pattern in Japanese is:

• 兄 = my older brother
• お兄さん / 兄さん = someone else’s older brother, or a more direct/casual way to address an older brother

So in a sentence like this, 兄は… sounds natural because the speaker is referring to their own older brother in plain narration.

は marks 兄 as the topic of the sentence.

So 兄は means something like:

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

It does not simply mean grammatical subject in the same way English does. It sets up older brother as the person the sentence is about.

Inside the sentence, other things can still be marked with other particles like が and に.

Because this part is describing where something does not exist, not where an action happens.

In Japanese:

• に is used with ある / いる / ない to mark the location where something exists or does not exist
• で is used for the place where an action takes place

Here the relevant part is:

• 洗面所に石けんがない
• There is no soap in the washroom

So 洗面所に is correct because it is the location of ない.

石けんがない presents soap as the thing that is absent.

With existence expressions like ある / いる / ない, が is very common for the thing that exists or does not exist.

So:

• 洗面所に石けんがない = There is no soap in the washroom

If you said 石けんはない, it would often sound more contrastive, like:

• As for soap, there isn’t any
  maybe implying something else is there

In this sentence, が is the most natural neutral choice.

こと turns the whole clause before it into a noun-like unit.

So:

• 石けんがない = there is no soap
• 石けんがないこと = the fact that there is no soap

This is needed because 気づく often takes a noun-like phrase as what someone notices.

So:

• 石けんがないことに気づく
• to notice that there is no soap

This is a very common pattern:

• [short clause] + ことに気づく
• notice that ...

Because 気づく commonly uses に for the thing noticed.

So the structure is:

• [something] に 気づく
• to notice [something]

Examples:

• 間違いに気づく = notice a mistake
• 変化に気づく = notice a change
• 石けんがないことに気づく = notice that there is no soap

So the second に belongs to 気づく, not to ない.

The て-form connects this action to the next one.

Here it means something like:

• He noticed that there was no soap, and then...
• After noticing that there was no soap, ...

So the flow is:

1. he noticed there was no soap
2. on the way back, he bought some and brought it

The て-form often links actions in sequence.

帰り means the return, the way back, or one’s trip home.

With に, 帰りに means:

• on the way back
• on the way home
• when returning

So in this sentence, it means he bought it during his return trip.

A similar pattern is:

• 行きに = on the way there
• 帰りに = on the way back

Usually it means on the way back / in the course of going back, not after arriving home.

So here the image is that the older brother noticed there was no soap, and then while he was out and coming back, he stopped somewhere and bought some.

If it meant clearly after he got home, Japanese would normally say it differently.

This is a compound expression with important nuance:

• 買って = bought
• きて = came (toward the speaker’s place / current situation) after doing that
• くれた = did it for the speaker’s benefit

So 買ってきてくれた means something like:

• he bought it and brought it back for me/us
• he went and bought it for me/us

This sounds much more natural than just 買った, because the soap was purchased elsewhere and then brought back.

The difference is the added sense of coming back with it.

• 買ってくれた = he bought it for me
• 買ってきてくれた = he bought it and brought it back for me / came back with it for me

The くる part often shows movement toward the speaker or toward the current place after the action.

In this sentence, that nuance fits very well: he noticed there was no soap, then bought some, then returned with it.

くれる shows that someone does something for the speaker or someone close to the speaker.

So 買ってきてくれた does not just mean he bought and brought it. It means:

• he kindly bought and brought it for me/us
• the action benefited the speaker’s side

This gives the sentence a personal and appreciative feeling.

The object is omitted because it is obvious from context.

Earlier the sentence says:

• 洗面所に石けんがない
• there was no soap in the washroom

So when the sentence continues with 買ってきてくれた, the listener naturally understands that he bought soap.

Japanese very often omits things that are clear from context.

Yes, that is the natural reading.

Because 兄は sets the topic, the rest is understood as describing what the older brother did:

• he noticed there was no soap
• he bought some on the way back
• he brought it for the speaker’s benefit

Japanese often avoids repeating the subject once it is established.

Not really in this sentence.

The location 洗面所に is attached to 石けんがない, so the meaning is specifically:

• there was no soap in the washroom

Without that location, the sentence could be more general. But here the place is clearly given.

That is very normal.

石けん can be written in different ways, including 石鹸, but the kanji are not very common in everyday writing because they are relatively difficult and less familiar to many readers.

So Japanese often writes it as:

• 石けん
• or せっけん

This is common and natural.

The common readings are:

• 兄 = あに
• 洗面所 = せんめんじょ
• 石けん = せっけん
• 気づいて = きづいて
• 帰り = かえり
• 買って = かって
• きて = きて
• くれた = くれた

So the whole sentence is read:

• あには せんめんじょに せっけんが ないことに きづいて、 かえりに かって きて くれた。

Yes. The biggest source of that nuance is くれた.

Without くれた, the sentence would be more neutral, just reporting what happened. With くれた, it suggests:

• the older brother did something helpful for the speaker
• the speaker likely feels appreciative

So the sentence does not just state a fact. It has a slight emotional tone of he was nice enough to do that for me/us.