Informal:
* Сестрёнка (sestrënka): This is the most common and affectionate way to say "little sister."
* Младшая сестра (mladshaya sestra): This literally translates to "younger sister," and is also informal.
Formal:
* Младшая сестра (mladshaya sestra): While this is also the informal translation, it can be used in formal contexts.
* Сестра (sestra): This is the general word for "sister" and can be used for both older and younger sisters in formal situations.
The best choice depends on your relationship with the person you are talking to and the tone of the conversation.