What would the motive have been? MOO.
Well, ok, things like money and jealousy/revenge don't spring into my mind with hogtying and encasement in concrete, jmo. Unless it's the mob, I know they're "into" the terrorize and degrade aspects to get their "message" across. I suppose the hogtying could have had some logistical advantage for the murderer, meaning it made the poor victim easier to transport, but still, this is an icy cold person used to killing, moo. This wasn't some act of passion, jmo, like someone insisting he was deeply in love, jilted, who went bananas, "heat of the moment" type thing (which is no less of a horror because the victim is no less dead). This is someone very cold, very calculating, intelligent, methodical, merciless. He knows how important it is to make that body "go away," and I mean really go away, and for decades. I read all about the family, yes, I see there's a hornet's nest there, and the father was dead by the time this happened, right? I think the family played a role in that they made this victim extremely attractive to this murderer, knowing he could ply, manipulate, and calculate her literally into an early grave with relaitve ease. I don't think this is someone you'd usually expect to see listed in a phone directory, I can see how that happened only with an alias. That girl was way, way too young to get wrapped up with a a man of that age to begin with. This guy's slick, he's cunning, and he puts on quite the act. I'm guessing he for the time came off as quite the catch in his own way. There are a lot of unknowns here because it's so long ago, but I'm guessing "Donald Grant" had several important pieces of "information" for the mother that were bold-faced lies. If the child was ever born, which I doubt, I would be extremely alarmed at the fate of that child at the hands of Mr. Alias Donald Grant.
The motive is he's full, full of hate, drawn to very, very young girls/has deep-rooted psych issues, and has figured out how to "express himself" in this manner without detection. It might be the mob, but I doubt it, they wouldn't go to the amount of trouble this guy went to, and to murder a girl of this age. I think that's a solo actor, I don't think it's the first time he killed and I'm sure it wasn't the last. It was undoubtedly the last time he was "Donald Grant," though. Also, I think this half-brother doled out too much info to be this guy, jmo. No way to know right now what happened, but my guess is this is an extremely calculating individual who kept right on killing. He likes it, he's a twisted individual with an axe in his head to grind. Was money a motive? Maybe played a role but this girl's background in no way screams "money," so if it had a role, it was a peripheral one, again, jmo. The way there are no pictures of her is bizarre to me, but is it possible the class photos didn't actually list names? I believe I've seen class photos from this era where the names of the individual students aren't listed. The lack of pics is odd, but I think that may have gone with the "shady" territory the family had maneuvered themselves into. And back in this era, it was nothing like it is now with photos, you couldn't take snaps on your phone & computer. You had to have a camera, you had to get the pics developed, a lot of people went to have portraits done with professional photographers. I wouldn't necessarily think they were planning to murder her all along. Again, though, at this stage, still too many unanswered questions to know, but right now, I think the bulk of those questions are answered only by knowing more about "Donald Grant."