The wife really was the weak point of this movie
She finds out that Charlie possibly killed his wife and that weird stain on the wall is the blast from the murder and she's like 'oh how do you know that's blood?'
And he explains that he got away with making it look like a suicide.
She's like "you don't know that" and proceeds to let him into the house, even after it's apparent that Charlie attempted to murder her husband on the road.
It would have worked better if she was suspicious about him as well and only let him into the house because she was afraid he'd kill her if she didn't.