* 赤ちゃん (akachan) の妹 (imōto): This is the most literal translation. 赤ちゃん (akachan) means "baby," and 妹 (imōto) means "younger sister." This is good if you want to emphasize that the sister is a baby.
* 小さい (chiisai) 妹 (imōto): This translates to "small younger sister." 小さい (chiisai) means "small." This is a good option if you want to emphasize the sister's age but not necessarily that she's an infant.
* ベイビーシスター (beibī shisutā): This is a loanword directly from English. While understandable, it might sound slightly unnatural or childish depending on the context.
The best choice will depend on the context. If you're talking about a very young infant sister, 赤ちゃん の妹 is the most appropriate. If the sister is a toddler or young child, 小さい妹 might be better. Using the English loanword is generally less preferred unless you are speaking casually with someone who is comfortable with English loanwords.