Erik's big brother died in the accident that happened before the beginning of the game
But he couldn't face it so he blocked it out the trauma. His family acts super uncomfortable whenever he's mentioned at all and his sister, Brunhilde even mentions having to face the truth eventually. His brother lives or lived up in the mountains where the accident took place. It could be possible they were merely visiting his brother in the days before the accident and he couldn't bring himself to see his brother afterwards but it could also be he physically couldn't as he'd past on. His oldest sister mentions that if he doesn't face the truth there's no point in him being at St. Dymphna's so it could have been her calling from his older brother's old cell number at the beginning to try and get through to him.Erik's diagnosis is post-traumatic stress disorder with conversion disorder ("hysterical paralysis") of one leg
Honestly, even with the clues given so far, it's not hard to figure out. Some sort of accident took place in the mountains, and ever since then, Erik experiences panic, flashbacks, numbness and increased anxiety (all symptoms of PTSD) and an on-again, off-again weakness of one leg (fairly typical of conversion disorder). Symptoms of conversion/dissociation are very common in PTSD and may give us a clue about the kind of accident he was involved in - perhaps his leg was trapped under something, perhaps his brother lost or broke a leg, perhaps he was driving at the time and caused an accident.