There's a scene where he and Sammy are standing outside, looking out on the night. A moth flutters by and lands on Ruffalo's arm. His eyes drop to it and he moves his arm, playing with it. With no way of predicting a moment like that, Ruffalo acts on pure instinct, and it's dead-on. Lonergan is brave enough to avoid pasting a rosy ending on his carefully woven movie.
Terry does do something thoughtlessly cruel, which throws the siblings' relationship into turmoil.
But that's where Lonergan shows his greatest strength, avoiding the easy way out. This isn't a movie about solving problems and tidying up messes. It's a movie about living through pain and holding onto family love, no matter what.