ane ha nodo ga itai toki, kasituki no mizu wo maiban kaeru you ni site imasu.

Because they are doing different jobs in the sentence.

• 姉は sets older sister as the topic: As for my older sister...
• のどが痛い means the throat hurts. Here, のど is the thing that has the painful condition, so が marks it.

This is very common with body-part expressions in Japanese:

• 頭が痛い = my head hurts / I have a headache
• お腹が痛い = my stomach hurts
• のどが痛い = my throat hurts

So the structure is basically:

• 姉は = as for my older sister
• のどが痛いとき = when her throat hurts
• 加湿器の水を毎晩替えるようにしています = she makes a point of changing the humidifier water every night

Japanese often describes physical discomfort as [body part] + が + adjective.

So instead of saying I have a sore throat, Japanese literally says something closer to:

• My throat is painful
• The throat hurts

That is why 痛い is used here. It is a very natural way to talk about pain in Japanese.

A few similar examples:

• 歯が痛い = my tooth hurts
• 目が痛い = my eyes hurt
• 腰が痛い = my lower back hurts

So のどが痛い is just the normal Japanese pattern.

とき means when or at the time when.

In this sentence, のどが痛いとき means:

• when her throat hurts
• when she has a sore throat

It connects the condition to the action that follows. So the sentence is saying that in the situation where her throat hurts, she changes the humidifier water every night.

A simple way to think of it:

• A とき、B = When A, B

Here:

• のどが痛いとき、加湿器の水を毎晩替える
  = When her throat hurts, she changes the humidifier water every night

This is a very common question.

With とき, the tense before it helps show the time relationship.

• 痛いとき = when it hurts / when her throat is hurting
• 痛かったとき = when it hurt / when her throat was hurting

In this sentence, the speaker is talking about a general habit: whenever her throat hurts, she changes the water every night. So 痛いとき fits well.

If you used 痛かったとき, it would sound more like you are referring to a specific past time or past situation.

So:

• のどが痛いとき、～しています = when her throat hurts, she does this
• natural for a repeated habit

ようにしています means something like:

• make a point of ...
• try to ...
• make it a habit to ...
• be sure to ...

So 替えるようにしています means:

• I/she try(ies) to change it
• I/she make(s) a habit of changing it
• I/she make(s) sure to change it

This pattern is very useful for talking about habits or conscious efforts.

Examples:

• 毎日運動するようにしています。
  = I try to exercise every day.

• 野菜をたくさん食べるようにしています。
  = I make a point of eating lots of vegetables.

In your sentence, it suggests this is not just a one-time action. It is something she intentionally does as a routine or precaution.

In the pattern dictionary form + ようにしている, Japanese normally uses している to show an ongoing habit or continued effort.

So:

• 替えるようにしています = I am making it a habit to replace it / I try to replace it
• 替えるようにします can sound more like I will try to do that or I’ll make sure to do that, often as a decision from now on

In other words:

• ～ようにしている = current habit / established practice
• ～ようにする = decide to do so / try to do so

So しています is the natural choice here because the sentence describes an existing routine.

Both can be read かえる, but they are used differently.

• 替える = to replace one thing with another
• 変える = to change/alter something

Here, the water in the humidifier is being replaced with fresh water, so 替える is the best choice.

Examples:

• 電池を替える = replace the batteries
• 水を替える = replace the water
• 予定を変える = change the schedule
• 考えを変える = change one’s mind

So 加湿器の水を替える means to replace the humidifier water.

It can be understood as the water in the humidifier or the water used for the humidifier.

Japanese AのB is very flexible. It can show many relationships, including:

• possession
• location-related association
• purpose
• category

Here, 加湿器の水 naturally means the water associated with the humidifier, especially the water inside it that you use and replace.

So in natural English, you would usually say:

• the humidifier water
• the water in the humidifier
• the water for the humidifier

毎晩 is an adverb meaning every night, so it describes when the action happens.

In Japanese, adverbs are often placed before the verb they modify, so:

• 加湿器の水を毎晩替える

is very natural.

You may also hear slightly different placements, such as:

• 毎晩、加湿器の水を替える
• 加湿器の水を、毎晩替える

These are all understandable, though the original order is very normal and smooth.

So the main point is:

• 毎晩 tells you the frequency/time of 替える
• its position is flexible, but it usually appears before the verb

Yes. The topic 姉は carries through the sentence.

Japanese often leaves out subjects and objects when they are already clear from context. After 姉は is introduced, the listener understands that the later action also refers to her unless something changes.

So the sentence is understood as:

• As for my older sister, when her throat hurts, she makes a point of changing the humidifier water every night.

Japanese does this constantly, so getting comfortable with omitted subjects is very important.

In a sentence like this, 姉 usually means my older sister when speaking from the speaker’s point of view.

Japanese family terms often depend on perspective:

• 姉 = my older sister / our older sister
• お姉さん = older sister, often when referring to someone else’s older sister or speaking more politely

That said, context matters. If the broader conversation is about someone else’s family, 姉 could refer to that person’s older sister in casual narration. But by default, many learners will understand 姉 here as my older sister.

Because only the sentence ending needs to be polite. Inside a sentence, clauses often stay in plain form.

So:

• 痛い is plain form before とき
• 替える is plain form before ようにしています
• しています is the polite ending

This is completely normal Japanese.

For example:

• 時間があるとき、勉強します。
  = When I have time, I study.

Here too:

• ある is plain
• します is polite

So the mix of plain forms inside the sentence and a polite ending is standard grammar, not inconsistency.

It usually suggests a conscious habit or effort, not necessarily a guaranteed 100% action every single time.

So 替えるようにしています is softer than a blunt 替えます.

It implies something like:

• she tries to change it every night
• she makes a point of changing it every night
• she makes sure to change it every night

Depending on context, it may sound very close to a regular habit, but the nuance is still intentional effort rather than a bare statement of fact.

So this pattern is great when you want to express:

• habit
• self-discipline
• health advice
• lifestyle effort