Ma sœur s’habille presque toujours pareil quand il pleut : un imperméable et des baskets noires.

Because s’habiller is a reflexive verb. It literally means to dress oneself / to get dressed.

• habiller quelqu’un = to dress someone
• s’habiller = to get dressed, to dress oneself

So:

• Ma sœur s’habille... = My sister gets dressed / dresses herself...

In French, clothing actions are very often expressed with reflexive verbs:

• se lever = to get up
• se laver = to wash oneself
• s’habiller = to get dressed

The full reflexive pronoun is se, but before a vowel sound it becomes s’. This is called elision.

So:

• se + habille → s’habille

You see this a lot in French:

• je aime → j’aime
• me appelle → m’appelle
• se habille → s’habille

It helps the sentence sound smoother.

Here, pareil means the same way or the same.

In s’habiller pareil, it means to dress the same way.

So the idea is:

• Ma sœur s’habille presque toujours pareil quand il pleut
  = My sister almost always dresses the same way when it rains

This is a very natural French expression:

• faire pareil = to do the same
• s’habiller pareil = to dress the same way

Because in this sentence pareil is not directly describing ma sœur. It is being used in an adverb-like way, meaning the same way.

So here:

• s’habiller pareil = to dress the same way

In that use, pareil normally stays unchanged.

Compare:

• Elle porte une robe pareille.
  Here pareille describes robe directly, so it agrees with the feminine noun.

But:

• Elle s’habille pareil.
  Here pareil means in the same way, so it does not change.

Presque toujours means almost always. It is an adverbial expression, and in French these often come after the conjugated verb.

So:

• Ma sœur s’habille presque toujours pareil...

This placement is very natural. French often puts frequency expressions like these after the verb:

• Il mange souvent ici. = He often eats here.
• Elle se couche toujours tôt. = She always goes to bed early.
• Ma sœur s’habille presque toujours pareil.

You do not usually translate word-for-word from English word order.

Here il does not refer to a person or thing. It is an impersonal subject, just like English it in it rains.

So:

• il pleut = it is raining / it rains

French weather expressions often use this impersonal il:

• il pleut = it rains
• il neige = it snows
• il fait froid = it is cold

So quand il pleut simply means when it rains.

Because the sentence is listing the clothes she usually wears:

• un imperméable = a raincoat → singular
• des baskets noires = black sneakers → plural

So the articles match the number:

• un for one masculine singular noun
• des for plural nouns

Also, noires agrees with baskets:

• baskets is feminine plural
• so noir becomes noires

Agreement pattern:

• masculine singular: noir
• feminine singular: noire
• masculine plural: noirs
• feminine plural: noires

Yes. In everyday French, des baskets commonly means sneakers / trainers.

This can surprise English speakers because basket in English usually means a container, but in French des baskets is a standard word for athletic shoes or casual sneakers.

Depending on region and context, you may also hear:

• des tennis
• des chaussures de sport

But des baskets is very common and natural.

The colon introduces an explanation or example of what comes before.

So the sentence first says:

• She almost always dresses the same way when it rains

Then after the colon, it specifies what that means:

• a raincoat and black sneakers

In English, this works similarly:

• She always wears the same thing when it rains: a raincoat and black sneakers.

So the colon is simply introducing the list of clothes.

Because sœur is a feminine noun, so the possessive adjective must also be feminine singular:

• ma sœur = my sister
• mon frère = my brother

French possessive adjectives agree with the noun possessed, not with the owner.

So:

• ma sœur = my sister
• mes sœurs = my sisters
• mon frère = my brother

A useful reminder:

• mon / ma / mes depend on the gender and number of the noun that follows.

Yes, depending on context.

Strictly speaking, s’habiller means to get dressed / to dress oneself. But in natural English, when talking about someone’s usual style, we often translate it as dresses.

So this French sentence could be understood as:

• My sister almost always gets dressed the same way when it rains or more naturally:
• My sister almost always dresses the same way when it rains

Both capture the idea well.

Sœur can be tricky because of the œ sound.

A rough guide:

• it sounds somewhat like the vowel in British English bird or fur, but not exactly
• the final r is the French r, produced farther back in the throat than in English

Approximate pronunciation:

• sœur ≈ sur / sirr with a French vowel and French r

The main thing to notice is that it is one syllable.

Also:

• the œ in sœur is the same spelling you see in words like cœur

Yes, but it would sound more formal or more explicit.

Compare:

• Elle s’habille pareil.
  = natural, everyday French

• Elle s’habille de la même façon.
  = more literal, more explicit: She dresses in the same way

In ordinary conversation, pareil is very common and idiomatic here. So the sentence sounds natural as written.