In Mainland, 小姐姐 is a new word (used mostly by the youth) to address a 'female young stranger' in a polite but causal way.You can understand it as a less formal version 'Miss''. She can be a bit older or younger than you, it doesn't matter. To most people, it means the other 'older girl'. Say you have a five. Your friend brings a 7-year-old girl when visiting you.Then you can tell your five to greet the older one by saying 'Have you said "hello" to 小姐姐'？ ——Here 7 is like an elder sister to 5, but only a bit older and 7 is much younger than you. So you add a 小（litter） before 姐姐（older sister） 小妹妹 somehow sounds weird. I agree with @ Teoh Zhi Hau 张志豪, it sounds like "an elderly person tries to flirt a younger female"

it's rare to hear a native speaker say "小姐姐“. If you're talking to a little girl and trying to remind her is old enough to act like a sister, then maybe I'll something like this '你已經是小姐姐囉！要做個好榜樣喔！“ Most of the time, we only say "小姐“, means "miss". "小妹妹“ is usually used on the elementary kids or younger. The teenager girl, I'd rather call them just "妹妹“. And the teasing situation will be another thing. Hope my answer helps ;)

We don't really use "小姐姐" as "小" means small/young and "姐姐" means elder sister, they actually contradict each another. The word "小妹妹" could imply a different meaning in different scenario, most of the time it means either "little sister" or "little girl", a way to address girls who aren't adult. However in native speaking this term could be also be heard when an elderly person tries to flirt a younger female by calling her "小妹妹", which I personally find it quite rude.

姐姐: Elder sister 妹妹: younger sister