I swear the onii-chan somehow pulled an Isekai Tensei Soudouki and just hangs out in her mind. There are a lot of places where the onii-chan is just mirroring what she does.
I wonder if something happened when her older brother died so, in reality, those flashbacks to her brother is actually just her brother talking to her, chillin' in her mind. Thinking about it on the lines of Isekai Tensei Soudouki