Responses
It´s the same in Japan with "onee-chan/-san" which doesn´t always mean that she´s her real sister.
It´s more like about wanting to have someone like her or something like that.
Like Ibuki x Saki in D4DJ for example, where the older Ibuki protects the younger Saki and the latter sees her as a "sister".
sorry but I cannot help but laugh at the comments thinking the use of ‘sister’ in this context means theyre related. I think originally it is either ‘mei mei’ or ‘jie jie’ wc is a common address in cn and if you translate it in english it is always the term sister. Similar to ‘gege’ wc is always being translated to brother (always leave a bitter taste in my mouth). English trans sometimes really fvkng sucks esp when the words being used in original language do not have eng equivalent so they tend to loose their meaning/misinterpret stuff