Personally I hope it is an existing character because I think the series is already starting to get a bit overloaded with Super Sleuth characters.
Consider also the possibility that the sibling could be a half-brother or step sibling, (either to Sera, Akai or both) which could explain lack of shared family names, allow for a less (or no) family resemeblence, and even allow for having grown up in different areas with different accents