For me that is one of the really interesting things about this storyline because I can understand where they are both coming from. I feel sorry for Charlie and I feel sorry for Ruby and you get why both of them are acting/have acted the way they do.
Ruby has just had everything ripped out from under her, her mother and father aren't who she thought they were, her sister is now someone she can't trust and her father is a rapist, she's bound to be confused and Grant is offering her an explanation that doesn't mean she's the product of something terrible that happened to Charlie, I think she believes it because she wants it to be true, she doesn't want to believe that she was concieved when Charlie was raped.
Charlie on the other hand has had keep everything to herself for 16 years, she has had to deny that she was Ruby's mother but has also had to live with the fact that she rapist is out there living his life while she's been living with the scars of his actions. This backstory explained a lot about Charlie and her previos actions. And now everyone knows she's being forced to confront it. She's not thinking like a cop at the moment, she's thinking like a terrified 14 year old whon was made to feel ashamed and denied any type of justice.