弟がいる

have a younger brother

→ 弟
phrase Rare #20,662
have a younger brother 弟 refers specifically to a younger brother. がいる literally describes someone as existing or being present, and is commonly used to say that someone has a younger brother.
JA: 私には弟がいる。
EN: I have a younger brother.

トムには建築家の弟がいる。

Tom has a younger brother who is an architect.

彼には弟がいる、そして東京に住んでいる。

He has a younger brother, and he lives in Tokyo.

トムは弟がいるの?

Does Tom have a younger brother?

隣の部屋に弟がいる。

My younger brother is in the next room.

私には双子の弟がいる。

I have a younger twin brother.

本を読んでいるが隣に弟がいる。

I'm reading a book, but my younger brother is next to me.

弟がいるなんて聞いてないよ。

You never told me you had a younger brother.

弟がいるなんて教えてくれたことなかったね。

You never told me you had a younger brother.