My thinking on what will tie Sebastian in to the main investigation is his digging into the American branch of the family, to try and find the current living heir of the Brotherton family; the book that clued him in to the American branch was a couple centuries old, so his mission in the states is following up on that geneology to find the more modern relatives. Perhaps he's at the asylum finding out whether the Brotherton descendant contained therein is [i]non corpus mentis[/i] and he needs to find someone else.