לאחותי יש אלרגיה לאבק, ולכן האף שלה סתום כמעט כל החורף.

Yes—literally, לאחותי is to my sister:

• ל־ = to
• אחות = sister
• ־י = my

So לאחותי = to my sister.

But Hebrew often expresses possession with the pattern:

ל־ + person + יש + thing
literally: to someone there is something
meaning: someone has something

So:

לאחותי יש אלרגיה לאבק
literally: To my sister there is an allergy to dust
natural English: My sister has a dust allergy / My sister is allergic to dust

In Hebrew, everyday possession is usually expressed with יש rather than with a direct verb like English have.

The basic pattern is:

• יש לי = I have
• יש לך = you have
• יש לו/לה = he/she has

So:

• לאחותי יש אלרגיה = my sister has an allergy

This is one of the most important basic Hebrew structures to learn.
The negative version is:

• אין לי = I don’t have

So Hebrew uses יש / אין for possession much more than English does.

Here, לאבק means to dust or more naturally to dust particles / dust in the sense required by allergy.

Hebrew often uses ל־ after certain nouns and adjectives where English would use to:

• אלרגיה ל... = an allergy to...

So:

• אלרגיה לאבק = an allergy to dust

This ל־ is not the same grammatical role as in לאחותי.
In לאחותי, it marks the possessor (to my sister).
In לאבק, it belongs to the phrase אלרגיה ל... (allergy to...).

Yes, that is grammatically possible.

Both of these work:

• לאחותי יש אלרגיה לאבק
• יש לאחותי אלרגיה לאבק

But לאחותי יש... is often more natural when the speaker is introducing the person as the topic: As for my sister, she has...

Starting with לאחותי can feel slightly more natural in conversation because it frames the sentence around my sister right away.

לכן means therefore, so, for that reason, or that’s why.

In this sentence:

לאחותי יש אלרגיה לאבק, ולכן האף שלה סתום...
= My sister has a dust allergy, and therefore / so her nose is blocked...

It connects the cause and result:

• cause: she has an allergy to dust
• result: her nose is blocked

In everyday English, so is often the closest translation.

Because with the separate possessive word שלה (hers / her), Hebrew usually makes the noun definite too.

So:

• האף שלה = her nose
• literally: the nose of hers

This is a very common Hebrew structure:

• הספר שלי = my book
• הבית שלהם = their house
• האף שלה = her nose

Without ה־, it would usually sound less natural here.

Yes. Hebrew has two ways to express possession here:

1. האף שלה = her nose
2. אפה = her nose (using a possessive suffix)

So a sentence like this is also possible:

לאחותי יש אלרגיה לאבק, ולכן אפה סתום כמעט כל החורף.

That said, in modern everyday Hebrew, the separate possessive form with של / שלי / שלה is often more common and clearer, especially in normal speech.

There is no separate word for is in the present tense in Hebrew.

So:

• האף שלה סתום
  literally: her nose blocked
• natural English: her nose is blocked

This is very normal Hebrew. In the present tense, Hebrew usually leaves out is / are / am.

Compare:

• הוא עייף = he is tired
• היא בבית = she is at home
• האף שלה סתום = her nose is blocked

Because סתום agrees with האף.

• אף is a masculine singular noun
• so the adjective is סתום = masculine singular

If the noun were feminine singular, the adjective would usually be סתומה.
If plural, it would change again.

Examples:

• האף סתום = the nose is blocked
• הדלת סתומה = the door is closed/blocked
• החלונות סתומים = the windows are shut

So this is standard adjective agreement.

It behaves like an adjective meaning blocked, stopped up, or stuffed.

In many cases, forms like סתום come historically from verb patterns, but for learners it is usually easiest to understand it here as a descriptive adjective:

• nose
  • blocked

So:

האף שלה סתום = her nose is blocked / stuffed up

In everyday usage, that is the important point.

כמעט כל החורף means almost the whole winter or for almost the entire winter.

Breakdown:

• כמעט = almost
• כל = all / the whole
• החורף = the winter

So literally:

almost all the winter

In Hebrew, time expressions often do not need a separate word for English for.

English says:

• Her nose is blocked for almost the whole winter

Hebrew can simply say:

• האף שלה סתום כמעט כל החורף

This is a very common Hebrew pattern with durations of time.

Because כל החורף means the whole winter / all winter.

Compare:

• בחורף = in winter
• כל החורף = all winter / the whole winter

The ה־ on החורף makes it definite: the winter.
With כל + definite noun, Hebrew often means the whole X:

• כל היום = all day
• כל השבוע = all week
• כל החורף = all winter

Adding כמעט gives:

• כמעט כל החורף = almost all winter

Because this sentence is talking about dust in a general sense, not a specific dust.

So:

• אלרגיה לאבק = an allergy to dust

This is like English when we say allergic to dust, not allergic to the dust.

If you said לאבק here, it means dust as a general substance/category. That is the normal choice.