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.

This is probably not what the faculty member wants. It would have been best to make the illustrations "work for hire," but, since she did not do that, she should clarify the situation 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 her 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.