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! :)