It sounds like they all went out to a cliff (Probably Parker's idea since he's a guilty lil fuk), something went wrong, and Mark's brother ended up dying and Walter got hurt. I think Herbert also says what happened to Mark's brother was Walter's fault which I'm kinda inclined to believe since Walter is lowkey unhinged. I'm wondering whether or not he yeeted a kid or if it was an accident.
Then there's the weird identity thing that kinda implies Walter yoinked his identity from his twin. Like, the weird scene with Talbot, Walter's name changes to Herbert backwards or something like that when he's being unhinged. And the dream in Parker's route where Dream Walter calls Walter "Herbert" and asks how long hes going to keep lying to himself when he denies being him.