Translation:The sister of Andrew is younger than the brother of Sebastian by one year.
We use "ca" when in a proposition, there is an equality report between situations. In this situation, Andrew's sister is younger than Sebastian's brother. This means that the situations are not equal, like, let's say "I am as smart as you are/Eu sunt deștept(sau la fel de deștept) ca tine.", where we use "ca" correctly in a translation, because the situations express an equal order.
So, in Romanian, one should use "decât" when there is a difference between two or more situations described in a sentence and "ca" when the sentence describes two situations of equal order. Sănătate și spor la învățat! :)
Oh, thanks a lot! In another comment section (can't find it now) people claimed that in reality many Romanians use "ca" in all cases. Is that so? Would it sound weird for natives if I were to use only "ca"?
It wouldn't sound weird. It is true, many Romanians use "ca" when sometimes "decât" would be correct. But I think it's important to speak a language correctly, not just to speak a language for the sake of communication.
younger = "mai tânăr"(if male) or "mai tânără"(if female) Example: Mihai is younger than Alex. (Mihai este mai tânăr decât Alex) Mihaela is younger than Alexandra. (Mihaela este mai tânără decât Alexandra)
Sora lui Andrei este mai mică ca fratele lui Sebastian cu un an. -> The sister of Andrew is younger than the brother of Sebastian by one year
you can use mica for younger, but why is "smaller than the ..." wrong?