Sampson's 1995 paper supports the position that Einiosaurus, Pachyrhinosaurus, and Anchelosaurus are all closely related. The other theory that has been put forward is that Einiosaurus is closely related to Styracosaurus and Centrosaurus. Part of the determining factor in each theory is the construction of the skull. All of the above animals have similar horny frills. The frill of Einiosaurus, Pachyrhinosaurus, and Anchelosaurus all have two large horns of bone that extend from the top central area of the skull frill. Styracosaurus however, has multiple horns extending from the frill and Centrosaurus possesses two small inward curving horns of bone at the top of the central area of the frill. Therefore, Einiosaurus can logically fit in either family group as an intermediate frill design between Pachrhinosaurus and Anchelosaurus and between Centrosaurus and Styracosaurus on the other side of the debate.
In terms of nasal horns, however, the debate changes a little bit. The horns of Styracosaurus then Centrosaurus, and then finally Einiosaurus, in this line of the debate, slowly began, and ultimately did, curve forward as the species evolved from one another. The horns on the other side of the debate start with Einiosaurus as the earliest species, and the nasal horns gradually were lost to a prominence like a shield boss much more than a horn covered bone. This change from horn to rough boss are endorsed by Horner's research as well as Sampson's paper. Supraorbital horns on all five mentioned dinosaurs are nominally present as either small knobs of bone or by being absent entirely from the skulls.
Given the arguments, it seems, and this is entirely my opinion, that both theories may actually be one theory. The nasal horn of Styracosaurus curves in a slightly posterior facing manner, whereas the Centrosaurus nasal horn faces somewhat forward. It makes sense, that the nasal horn of Einiosaurus curving even more forward, which must have had some special reasoning or function, could certainly come next in this line. Continued evolution could certainly then follow into the next proposed line where the nasal horns adapted into the rough bosses of the Pachyrhinosaurus and then Anchelosaurus as well. It's just a thought, but I think it is one worth looking into if no one has before, and since I have not found a paper that says it has been looked at, it certainly should be looked at.