Liar's Edge (1992)

This Time, Shannon's Sleeping with the Enemy.

A child is traumatized when his father, a stuntman, dies in an attempt to go over Niagara Falls. Later, his mother meets and marries a truck driver, and things seem to be going OK. Then the new husband's creepy brother shows up, and the boy begins to get a sense that neither of the brothers is quite what they claim to be.