After Eric and Chris' mother was killed (by who is heavily implied to be Mira following the fateful placement of the Snail), their father turned into an alcoholic who regularly beat them with Eric receiving the brunt of the punishment. This dysfunction and abuse led Eric to becoming traumatized, explaining his panicked and emotional personality during the game.
The situation climaxed when Chris was killed by his father - it is implied that their father drowned Chris in a bathtub as punishment for misbehaving while Eric witnessed this. Eric then dumped his brother's body into a lake, leaving a traumatic emotional scar on his psyche.
Zero Time Dilemma
After Q-Team decides to inject Radical-6, Eric talks to Mira and Sean about his family in the lounge. Eric tells Sean that Chris is "about your age more or less".