Consider a system whereby any VP with exclusively third person arguments form your average nominative-accusative (or even erg-abs or whatever) structure. However, whenever a first or second person is involved, the system is inverse instead. Thus, the inverse and the direct markers also are, in some sense, a person marker: they signal the presence of at least a first- or second-person participant in the VP.
However, let's imagine further that even the presence of an indirect object in the first or second person triggers this. Let's, however, still have the inverse alignment be the only role-assigning marker even in that case.
