Moja siostra jest architektką, a jej koleżanka jest pielęgniarką.

Because after jest in this kind of sentence, Polish normally uses the instrumental case when saying what someone is by profession, role, or identity.

So:

• architektka → architektką
• pielęgniarka → pielęgniarką

This is very common with być (to be):

• Ona jest nauczycielką. = She is a teacher.
• On jest lekarzem. = He is a doctor.

So in your sentence, the -ą ending shows the feminine singular instrumental form.

They are two different possessive words:

• moja = my
• jej = her

Moja behaves like an adjective, so it changes form to match the noun’s gender, number, and case. Since siostra is feminine singular nominative, we get moja siostra.

Jej is much less variable in everyday Polish. As a possessive form, it usually stays jej regardless of the noun that follows:

• jej siostra
• jej brat
• jej koleżanka
• jej książki

So moja changes, but jej usually does not.

Koleżanka can mean a female friend, female colleague, female classmate, or generally a woman someone knows in a social, school, or work context.

It is broader than a single English word. The exact meaning depends on context.

A useful comparison:

• koleżanka = female friend/acquaintance/colleague/classmate
• przyjaciółka = close female friend

So jej koleżanka does not necessarily mean a very close friend. It could simply be her friend or her colleague, depending on the situation.

Both a and i can often be translated as and, but they are not used in exactly the same way.

In this sentence, a is natural because it connects two parallel pieces of information and gives a slight contrast or comparison:

• My sister is an architect, and her friend is a nurse.

So a here feels a bit like and, but also slightly like while or whereas.

Compare:

• Moja siostra jest architektką, a jej koleżanka jest pielęgniarką.
  My sister is an architect, while/her friend is a nurse.

Using i would sound more like simple addition, with less contrast.

Yes, very often it can.

You can say:

• Moja siostra jest architektką, a jej koleżanka jest pielęgniarką.

But also:

• Moja siostra jest architektką, a jej koleżanka pielęgniarką.

Omitting the second jest is common and natural when the meaning is still clear. The full version is also completely correct.

Because the sentence is about two women:

• siostra = sister
• koleżanka = female friend/colleague

So the profession nouns also appear in feminine forms:

• architektka = female architect
• pielęgniarka = female nurse

If you were talking about men, you would normally use masculine forms:

• Mój brat jest architektem.
• Jego kolega jest pielęgniarzem.

Polish usually marks gender more explicitly than English does.

They are in the nominative case, which is the normal case for the subject of a sentence.

Here is the structure:

• Moja siostra = subject
• jest = verb
• architektką = complement in the instrumental case

And then again:

• jej koleżanka = subject
• jest = verb
• pielęgniarką = complement in the instrumental case

So even though English relies heavily on word order, Polish also uses case endings to show what role words play.

Polish word order is more flexible than English word order, because cases show grammatical function.

The sentence as given is the most neutral:

• Moja siostra jest architektką, a jej koleżanka jest pielęgniarką.

But other orders are possible for emphasis, for example:

• Architektką jest moja siostra, a pielęgniarką jej koleżanka.

That version sounds more marked and emphasizes the professions.

So yes, the word order can change, but the original sentence is the most straightforward choice for a learner.

Because a is joining two separate clauses, each with its own subject and verb:

• Moja siostra jest architektką
• jej koleżanka jest pielęgniarką

In Polish, a comma is normally used before a when it connects clauses like this.

So the comma here is standard punctuation.

The letter ą is a nasal vowel. At the end of a word, many learners can safely think of it as sounding roughly like an o with a nasal quality.

So:

• architektką
• pielęgniarką

Do not pronounce the final ą like a plain English a.

A practical learner tip: if you cannot produce the exact nasal sound yet, an approximate nasal o is much better than reading it as a normal a.

In modern Polish, feminine profession forms like architektka and pielęgniarka are very normal when referring to women, and in this sentence they are the expected forms.

You may sometimes encounter variation in real life with some profession names, especially in formal, older, or profession-specific contexts, but for this sentence:

• siostra → architektką
• koleżanka → pielęgniarką

These feminine forms are the natural ones to learn and use.