君の兄さん

your older brother

→ 君
phrase Rare #34,345
your older brother 君の means “your,” and 兄さん is a familiar or polite way to say “older brother.” 君 can sound informal or familiar depending on the relationship.
JA: 君の兄さんは大学生ですか?
EN: Is your older brother a college student?

はじめ彼を君の兄さんと思っていました

At first, I thought he was your older brother.

違いがある分からないけど君の兄さん悪い事をした反省していた

I don’t know if there’s any difference, but your older brother said he regretted what he had done.

— Article 15 (2019)

君の兄さんニハールレイプしたんだ

Your older brother, Nihal, raped someone too.

— Article 15 (2019)

君の兄さんとても怒っている

Your older brother is very angry.

君の兄さん道で偶然会ったんだ

I ran into your older brother on the street.