Thanks for the feedback WhimsiCat and you make a good point. I agree that it's true that that is part of the meaning, but at the same time the use of Ani (older brother) rather than san is also rather informal and Kirihara isn't particularly close to Miyuki, nor is she present, so making such a distinction isn't necessary in the context. As such simply referencing he's the older Shiba doesn't convey the meaning of the change in honorifics which indicate he's being very casual and friendly which is why I went for brother in the end as given her absence the focus to me was more on the casual friendliness between them than the literal fact he's the older Shiba (I'd likely have reversed it and done as you suggested were Miyuki present).