I think so, too. It's not an uncommon phenomenon. The human mind is capable of compartmentalization to the point where severely traumatized people will completely "lose" (block) their memories of certain events.
We are observing only mere glimpses into BM's mindset and he's been pretty consistent with the "I'm innocent/grief-stricken/under immense stress not knowing what has happened to my wife or where she is/poor me, the victim/I'm being set up by LE to cover their incompetence". He will be amplifying these messages on a loop to his daughters. It's akin to a kind of brainwashing.
Something happened to fracture the extended Morphew-Moorman family communications two or so years ago. We don't know what that was, and it might not be of any huge significance in the bigger picture. It will, however, make it more difficult for the Morphew girls to reach out to the Moorman side of their family right now. BM convincing his daughters that the three of them need to support one another (at the expense of all other family connections) will be one of his priorities, the corollary being that if they did talk to anyone else, they'd be "betraying" their father and their tight family unit.
It's very possible that the daughters are currently angry at the "unfair" treatment LE is dishing out by setting their father up. Brainwashing has that effect. They are still viewing the world through their parent/father's lens and - due in part to the pandemic - have had little respite from that. They are isolated, with little opportunity for reality check challenges via work colleagues, friends, school associates, and other family members.
And he's all they have now (as they see it), so it's unsurprising that their traumatized minds are denying/blocking what is far too painful to contemplate.
The dam will breach, eventually. I think that the major search being organized by AM could well be a catalyst for that to happen. It will force the daughters to see things they haven't wanted to look at before. I suspect that the tight-knit family unit will begin to fracture at that point- if it hasn't already started doing so.