Moja siostra jest ode mnie starsza o trzy lata, a mój brat jest młodszy o rok.

Because Polish possessive words like my have to agree with the noun they describe.

• siostra is a feminine noun, so you use moja
• brat is a masculine noun, so you use mój

So:

• moja siostra = my sister
• mój brat = my brother

This agreement is very important in Polish and happens with adjectives too.

For the same reason: adjectives in Polish agree with the noun’s gender.

• siostra is feminine, so starsza
• brat is masculine, so młodszy

Compare:

• starszy brat = older brother
• starsza siostra = older sister
• młodszy brat = younger brother
• młodsza siostra = younger sister

So the adjective ending changes depending on who or what is being described.

Here ode mnie means than me / than I am after a comparative adjective.

So:

• starsza ode mnie = older than me
• młodszy ode mnie = younger than me

Literally, od/ode often means from, but after comparatives it is used to mark the thing or person being compared.

There are two things going on here:

1. od requires the genitive case, so you cannot use ja

   • ja = nominative
   • mnie = genitive/accusative form

2. Before mnie, Polish normally uses ode instead of od for easier pronunciation

   • ode mnie is the standard phrase

So ode mnie is the natural correct form.

Yes, you can use niż for comparisons, but the patterns are a little different.

You may hear:

• Moja siostra jest starsza ode mnie.
• Moja siostra jest starsza niż ja jestem.

In everyday speech, people also say:

• starsza niż ja

But for learners, ode mnie is often the easiest and most reliable pattern after comparative adjectives like starszy, młodszy, lepszy, gorszy, and so on.

This tells you by how much the ages differ.

• starsza o trzy lata = older by three years
• młodszy o rok = younger by a year

In Polish, o + accusative is often used to express the amount of difference:

• wyższy o 10 centymetrów = 10 centimeters taller
• droższy o 5 złotych = 5 złoty more expensive

So in your sentence, o trzy lata and o rok show the size of the age gap.

This is because Polish treats numbers and time-unit nouns in different ways.

• after trzy, you get lata
• after one, you usually just use the singular noun: rok

So:

• o trzy lata = by three years
• o rok = by a year

You can say o jeden rok, but o rok is more natural here unless you want special emphasis.

Also, Polish number patterns are tricky:

• 1 rok
• 2, 3, 4 lata
• 5+ lat

So you would say:

• o rok
• o dwa lata
• o pięć lat

Because a often links two statements that are related but contrasted.

Here the contrast is:

• the sister is older
• the brother is younger

So a works well as and, while, or whereas depending on context.

• Moja siostra jest starsza..., a mój brat jest młodszy...

If you used i, it would sound more like simple addition with less contrast. A is the more natural choice here.

Yes, you can. Polish word order is fairly flexible.

Both of these are natural:

• Moja siostra jest ode mnie starsza o trzy lata
• Moja siostra jest starsza ode mnie o trzy lata

The second version may feel a bit more neutral to many learners, but the first is also correct. Word order in Polish often changes emphasis rather than basic meaning.

So you should learn the structure, but not assume there is only one possible word order.

In normal standard Polish, yes.

• Moja siostra jest...
• mój brat jest...

Polish usually keeps jest in present-tense sentences like this. Leaving it out would sound incomplete, very informal, or stylistically marked.

So for learners, it is best to include it.

Because in Polish, when a joins two clauses, a comma is normally required.

Here you have two full clauses:

• Moja siostra jest ode mnie starsza o trzy lata
• mój brat jest młodszy o rok

Since they are linked by a, Polish punctuation puts a comma before it.

This is standard Polish punctuation, even where English might sometimes be less strict.