Re: Cracked Plaster Wall
The original plasterer did not have access to expanded wire lath. When it became available it was universally adopted because it prevented what you have there. The wire lath reinforces the inherently weak 90 degree joints. Plaster by itself is very low tensile strength.
I would fill the corner with expanding foam, and mud/tape it with a preformed inside corner (metal with paper fused to it) not corner bead!
Remove not the ancient landmark, which your fathers have set.