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 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 share an intention that he will be a joint owner.

If that's not what the faculty member wants and it is too late or otherwise impossible to make the work a work for hire, she should clarify her intentions by having her brother-in-law sign a copyright assignment. This is a contract that can be signed at any time to transfer the copyright from the author/artist to someone else. This won't make the new owner an author under the law, as the work for hire doctrine would have, but it's the second best thing - she will be a copyright owner by way of assignment.