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"