When would aniki be used between actual siblings?

I'm working on a story and my character's relationship is as such:

Character 1 is a boy. His parents divorced and his mother remarried. Character 2 is the daughter of that new marriage. She is a bit of a delinquent/sukeban.

Would she only call Character 1 oni-san/nii-san? Or could she use aniki?

What would be people's perception of their relationship if she used aniki?