Board Thread:Episode Discussions/@comment-3261234-20150416133517/@comment-2140588-20150416170402

But isn't "aniki" partially the same to "onii-san", yet it's a honorific not necessarily for a relative (older brother)?

Edit:  Onee-sama: a young girl using the term to address an older, unrelated girl could be hinting at a deeper relationship between the two of them.