You shouldn't hide the identity of someone's parent from them.
Kelly was raised by a single mother and never met her father.
Kelly's mother wants to protect Kelly.
Kelly's mother tells Kelly who her father is and lets Kelly decide what to do with that information.
Kelly's mother refuses to tell Kelly who her father is because he had an affair walked out on them.
Kelly is grateful to her mother and chooses not to contact her father to avoid hurting her mother.
Kelly's relationship with her mother deteriorates because Kelly's mother is not taking her feelings seriously.