The unexpected guest

When a stranger runs his car into a ditch in dense fog and makes his way to an isolated house, he discovers a woman standing over the dead body of her wheelchair-bound husband, with a gun in her hand. She admits to murder, and the unexpected guest offers to help her concoct a cover story. But is it possible that Laura Warwick did not commit the murder after all? If so, whom is she shielding? Perhaps the victim's retarded younger brother, his dying matriarchal mother or Laura's lover? The house seems full of possible suspects.