Here's how you might express it:
* Scottish Gaelic: You'd likely use a phrase like `bràthair beag` (brah-er beg), meaning "little brother." While this doesn't explicitly say *baby*, the word `beag` (beg) implies smallness, often suggesting youth. You could add a term of endearment depending on the context.
* Irish Gaelic: Similarly, you'd use `deartháir beag` (der-har beg), meaning "little brother." Again, `beag` (beg) implies smallness.
To emphasize the "baby" aspect, you could add words like:
* Scottish Gaelic: `bràthair beag bìodach` (brah-er beg bee-och) - "tiny/little baby brother" ( adding bìodach for extra emphasis on smallness)
* Irish Gaelic: `deartháir beag bídeach` (der-har beg bee-och) - "tiny/little baby brother" (adding bídeach for extra emphasis on smallness).
Ultimately, the best translation depends on the specific context and the age of the baby brother. `Little brother` is the closest and most natural translation in most situations.