La meva germana ha de rebre un missatge de la metgessa abans de cancel·lar la cita.

In standard Catalan, possessives normally go with a definite article:

• la meva germana = my sister
• el meu cotxe = my car

So the usual pattern is:

article + possessive + noun

Leaving out the article is generally not standard in this kind of sentence, though you may hear shorter forms in older, literary, or regional usage.

Ha de rebre means has to receive or must receive.

It is the Catalan structure:

haver de + infinitive = to have to / must

So here:

• ha = has (3rd person singular of haver)
• de rebre = to receive

Together: ha de rebre = has to receive

Yes, ha can be part of the present perfect, as in:

• ha rebut = has received

But in this sentence, it is not a perfect tense. It is part of the obligation structure:

• ha de rebre = has to receive
• not ha rebut = has received

So you can tell the difference by what follows:

• ha + past participle → perfect tense
• ha de + infinitive → obligation / necessity

Because after haver de, Catalan uses the infinitive.

So the pattern is:

• he de fer = I have to do
• has de venir = you have to come
• ha de rebre = she has to receive

This is similar to English have to + base verb.

Here it means from the doctor.

The preposition de can mean several things in Catalan, including of, from, or sometimes other related meanings depending on context.

In this sentence:

• un missatge de la metgessa = a message from the doctor

So the doctor is the sender or source of the message.

Because metgessa is the feminine form, meaning female doctor.

• metge = male doctor / doctor in masculine form
• metgessa = female doctor

So the sentence specifically refers to a woman doctor.

Because abans de is the normal structure before an infinitive:

• abans de sortir = before leaving
• abans de menjar = before eating
• abans de cancel·lar la cita = before canceling the appointment

So de is required here before the infinitive cancel·lar.

After abans de, Catalan often uses the infinitive when no separate subject is stated.

So:

• abans de cancel·lar la cita = before canceling the appointment

This is very natural when the subject is understood from context.

If you wanted a full clause with its own verb, Catalan would usually use something like abans que..., for example:

• abans que cancel·li la cita = before she cancels the appointment

Normally, the understood subject is la meva germana.

So the sentence is naturally read as:

My sister has to receive a message from the doctor before she cancels the appointment.

Because the infinitive cancel·lar has no explicit subject, Catalan usually lets context supply it. If you wanted to make a different subject clear, you would normally rephrase the sentence.

Because Catalan normally uses an article with a specific countable noun like this.

• la cita = the appointment

Here it refers to a particular appointment already understood in context, so the definite article is used.

Because Catalan only contracts certain combinations.

For example:

• de + el = del
• a + el = al

But with la, there is no contraction:

• de la
• a la

So de la metgessa is exactly what you should expect.

That dot is called the punt volat.

It appears in l·l to show that these are two separate l sounds, not the Catalan ll sound.

So:

• ll is one sound
• l·l shows a doubled l

This is a special spelling feature of Catalan, and cancel·lar is a very common example.

Yes. Catalan is flexible enough that you could also say:

Abans de cancel·lar la cita, la meva germana ha de rebre un missatge de la metgessa.

That means the same thing. The original version is completely natural; this alternative just puts the time condition first for emphasis.