Let's answer this by looking at the brother-in-law's illustrations. Are they
works for hire under the law? Probably not, because there's no mention of a
contract signed by both the faculty member and the brother-in-law before the
work began and it's not clear whether the work would fit into any of those
nine categories. The brother-in-law probably is not the faculty member's
employee either. That means he ownes his contribution and he will be a joint owner if he is contributing
copyrightable expression, intends that
expression to be merged into the unified work as a whole, and he and the faculty member both intend that he will be a joint owner.