A casual, familiar form of 'onee-san' meaning 'older sister'; commonly used by children or in close relationships to refer to an older sister.
「お姉ちゃん、宿題手伝って！」と弟が頼んだ。
"Big sis, help me with my homework!" the younger brother asked.
うちのお姉ちゃんは大学で経済学を勉強している。
My older sister is studying economics at university.
子どもたちは遊びの中でお互いを『お姉ちゃん』と呼び合っていた。
The children were calling each other 'onee-chan' while playing.
Used colloquially as a friendly form of address for a young woman or lass; can be a casual call to attract attention (nuance varies by region and speaker).
商店で、おばあさんが若い店員さんに『お姉ちゃん、ちょっといいかい』と言った。
At the shop, an elderly woman said to a young clerk, 'Hey miss, can I ask you something?'
道で『お姉ちゃん！』と呼び止められて驚いた。
I was surprised to be called 'onee-chan!' and stopped on the street.