senmenjo no kagami no mae de kami o naoshite iru to, imouto ga 「hayaku shite」 to itta.

の links nouns together, a bit like of or an apostrophe-s in English, depending on context.

• 洗面所の鏡 = the washroom/bathroom mirror
• 鏡の前 = the front of the mirror / in front of the mirror

So:

• 洗面所の鏡の前 = in front of the washroom mirror

Japanese often builds long noun phrases this way, from left to right.

Because で marks the place where an action happens.

In this sentence, the action is 髪を直している “fixing one’s hair,” and that action takes place in front of the mirror, so で is natural:

• 鏡の前で髪を直す = fix one’s hair in front of the mirror

By contrast, に is more often used for:

• destination: 学校に行く
• existence: 机の上に本がある
• specific point in time (in some cases)

So here, で is the location of the activity.

Yes. 直す has a broad meaning: to fix, correct, repair, straighten, put in order.

With 髪 it often means:

• fix one’s hair
• tidy one’s hair
• adjust one’s hairstyle

So 髪を直している means the person is in the middle of fixing/tidying their hair.

The structure is:

• 直す = to fix
• 直している = is fixing / has been fixing / is in the process of fixing

Because 髪 is the direct object of 直す.

• 髪 = hair
• を = marks the thing being acted on
• 直す = to fix/tidy

So:

• 髪を直す = to fix one’s hair

Even though in English we often say do my hair, Japanese uses 髪を直す very naturally.

Here ～ている shows an ongoing action, similar to the English progressive is doing.

• 直す = fix
• 直している = is fixing

So the person was in the middle of fixing their hair.

In Japanese, ～ている can also sometimes show a resulting state, but in this sentence it is most naturally the progressive: an action currently in progress.

Yes, but here it is being used in a temporal sense: when / while / just as.

So:

• 髪を直していると、妹が…言った = While I was fixing my hair, my little sister said... = When I was fixing my hair, my little sister said...

This と often describes something that happens as a natural or immediate occurrence connected to the first action.

A useful way to understand it here is:

• doing X, then Y happened
• when I was doing X, Y happened

Those are all possible patterns in Japanese, but they have slightly different feels.

• 直していると
  Often feels like: while I was doing that, then this happened.
  It can sound more immediate or event-like.

• 直していたとき
  More straightforwardly: when I was fixing my hair.

• 直していたら
  Often means something like: while I was doing that / when I did that, then..., sometimes with a sense of discovery or unexpected result.

So ～ていると is perfectly natural here and gives a nice “while I was doing that, this happened” feeling.

Because Japanese does not always match English tense the same way.

The main event is narrated in the past:

• 妹が…言った = my sister said

But the earlier clause describes the situation at that time:

• 髪を直していると = while I’m in the state of fixing my hair / while fixing my hair

Japanese often uses ～ている in subordinate clauses like this to describe the ongoing action that was happening when the past event occurred.

You may also hear 直していたとき in similar situations, but 直していると is natural and idiomatic.

Because と is the standard quotative particle used before verbs like:

• 言う = say
• 思う = think
• 聞く = ask/hear

So:

• 「早くして」と言った = said, Hurry up
• literally: said “Hurry up”

This is one of the most common uses of と in Japanese.

Literally, it is built from:

• 早い = early / fast
• 早く = early / quickly (adverb form)
• して = te-form of する

So word-for-word it looks like do it quickly.

But as a set expression, 早くして is a very common way to tell someone:

• Hurry up
• Come on, be quick
• Get a move on

It does not need an explicit object. The listener is expected to understand “do whatever you’re doing more quickly.”

It is the te-form, but in direct speech the te-form can function as a casual request or a somewhat pushy command, depending on tone.

So 早くして can sound like:

• Hurry up
• Come on, hurry up

It is less formal than:

• 早くしてください = please hurry up

Because this is speech from a younger sister, the casual form fits well.

が marks 妹 as the subject of 言った.

• 妹が言った = it was my younger sister who said it / my younger sister said it

Using が is very natural when introducing the person who performed the action in that moment.

If you used 妹は, it would sound more like you were setting my younger sister up as the topic, possibly with contrast or a broader discussion about her.

Here, が simply presents her as the one who spoke.

The subject is omitted because Japanese often leaves out information that is understood from context.

So 髪を直している does not explicitly say:

• I was fixing my hair
• she was fixing her hair
• he was fixing his hair

But from the overall sentence, the natural interpretation is I was fixing my hair, because the sister is the one speaking to that person.

This kind of omission is extremely common in Japanese.

Not exactly in every situation.

洗面所 usually refers to a place for washing up, such as:

• washroom
• powder room
• sink area
• bathroom wash area

Depending on the house or context, the best English translation might be:

• bathroom
• washroom
• sink area

The important grammar point is that it is the place connected to the mirror:

• 洗面所の鏡 = the mirror in the washroom/bathroom area

Not necessarily. It is usually broader than that.

The sister is probably not saying Fix your hair quickly in such a literal, narrow sense. More naturally, she means something like:

• Hurry up
• Get ready faster
• Come on, finish up

So 早くして refers to the whole situation, not just the act of touching the hair.

This is a very common narrative pattern in Japanese:

• [while/when doing X], [someone] did Y

Here:

• 洗面所の鏡の前で髪を直していると、 = while I was fixing my hair in front of the washroom mirror,
• 妹が「早くして」と言った。 = my younger sister said, Hurry up.

The comma after と helps show the shift from the background action to the new event.

It is a natural storytelling structure: first the scene, then what happened.