I have a much younger brother.

2025年9月5日

A: I need to buy some food because my brother is coming to visit this weekend.
B: Your brother? I’m surprised; you never mentioned having a brother.
A: Oh, didn’t mention that? I have a much younger brother. He’s 18 years old and he’s coming to check out the universities that he wants to get into.
B: You must be looking forward to it!

In this situation, what should you say ‘I have a much younger brother’ in Japanese?

Casual

A：あれ、言いってなかった？年としの離はなれた弟おとうとがいて、今いま18歳さい。志望しぼう大学だいがくを見みに来くるって。

Note: This is not a word-for-word translation.

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A word-for-word translation

A：あれ、言いっていませんでしたか？私わたしにはずっと年下とししたの弟おとうとがいるの。18歳さいで、志望しぼう大学だいがくを見みに来くるの。

I don’t think it sounds weird, and I feel like I’ve heard ずっと年下とししたの before. However, I find it a bit off.

Maybe it’s because the word 弟おとうと originally refers to a younger,
( 年下としした) sibling, so it feels a bit strange to specify 年下としした.

However, by adding ずっと, I feel like the awkwardness is alleviated a little, though.

In that sense, 年としの離はなれた is better for avoiding the weird connotations.
Maybe you feel 年としの離はなれた doesn’t contain the meaning ‘-er’ of ‘younger’, but in this case, 離はなれた indicates that meaning.

Siblings are basically apart from in age expect for twins.
So deliberately mentioning 離はなれた like this can convey the idea that it must be far apart.