Not only is Cousin Henry a comedy, it’s Old Comedy, as practiced by Aristophanes, among others.Old Comedy opens with the imposition of an arbitrary law or regulation that upsets the world of the story. In this case, Uncle Indefer’s insistence on leaving the property to his oldest male heir instead of his all-but-adopted niece results in an impoverished Isabel, a miserable Henry, angry tenants and an exodus of faithful servants. The resolution of Old Comedy is the repeal of the ridiculous rule and the return of society to its natural state. In Cousin Henry, the law is actually repealed before its effects begin to unfold. More unusual, I think, is the fact that a minor character (Apjohn the lawyer) is the agent of resolution.
I wonder whether Trollope was consciously following the Old Comedy formula. Quite possibly not. As literary critic Northrop Frye pointed out, literature tends to take a handful of archetypal forms. In any case, the plot here is merely a peg on which to hang a character study.