En juin, ma sœur réserve déjà ses vacances, parce qu’elle sait qu’en juillet tout coûte plus cher.

With months, French normally uses en:

• en juin
• en juillet
• en août

You do not usually put an article before the month in this kind of expression. So French says en juin, not dans le juin or au juin.

Because sœur is a feminine singular noun, so the possessive adjective is ma.

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

So:

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

Here, réserve comes from réserver, which means to book or to reserve.

So ma sœur réserve ses vacances means that she is booking / arranging her vacation.

Depending on context, réserver can be used for many things:

• réserver un hôtel = book a hotel
• réserver une table = book a table

In French, short adverbs like déjà, souvent, toujours, and bien often come after the conjugated verb.

So:

• elle réserve déjà
• il sait déjà
• nous arrivons souvent

That is why French says réserve déjà, not usually déjà réserve.

Because vacances is plural in French.

The possessive adjective must agree with the noun:

• sa for feminine singular
• ses for plural

So:

• sa voiture = her car
• ses vacances = her vacation / holidays

Even though in English vacation is often singular, French normally uses vacances in the plural.

In French, vacances is almost always used in the plural when talking about a holiday or vacation period.

So French says:

• Je pars en vacances.
• Ils réservent leurs vacances.

The singular vacance exists, but it usually means something different, like a vacancy or an empty position, not a holiday.

French often uses the present tense where English also uses the present, especially for:

• habits
• general truths
• things seen as current or scheduled

In this sentence:

• réserve can describe what she does at that time
• sait is a present fact
• tout coûte plus cher is a general truth

French also often uses the present when the time is clear from the context, even if English might sometimes think of it as a future idea.

Because que becomes qu’ before a vowel sound.

So:

• que elle → qu’elle
• que il → qu’il
• que en → qu’en

This is called elision. French often drops the final e of a small word before a vowel to make pronunciation smoother.

The verb savoir can be followed by a clause introduced by que:

• elle sait que... = she knows that...

Then the next word is en, which begins with a vowel, so que becomes qu’:

• elle sait que en juillet... → elle sait qu’en juillet...

So qu’en is just que + en with elision.

Here, tout means everything.

As a pronoun meaning everything, tout is treated as singular, so the verb is singular too:

• tout coûte
• tout va bien
• tout change

Tous usually means all or everyone, and it works differently.

So in this sentence:

• tout coûte plus cher = everything costs more

Because after the verb coûter, cher behaves like an adverbial expression, not like a normal adjective agreeing with a noun.

So you say:

• ça coûte cher
• tout coûte plus cher
• les hôtels coûtent cher

Even with a plural subject, cher usually stays unchanged in this structure.

Yes. That would also be correct.

French can place the time expression in different spots:

• elle sait qu’en juillet tout coûte plus cher
• elle sait que tout coûte plus cher en juillet

Both are natural. The version in your sentence puts en juillet earlier for emphasis on the time.

The comma separates the main idea from the explanation:

• ma sœur réserve déjà ses vacances
• parce qu’elle sait...

In French, punctuation can help make a sentence easier to read, especially when one clause explains the other. The comma here is natural, though punctuation can sometimes vary depending on style.

It can mean her, his, or its.

French possessive adjectives do not show the gender of the owner. They show the number and gender of the thing possessed.

So:

• ses vacances can mean her vacation, his vacation, or its holidays depending on context

Here, because the subject is ma sœur, we understand ses as her.