I loved all the Charlie/Ruby storylines surrounding Ruby's true heritage and Charlie's rape. I agree that the groundwork was always there with Charlie's character for something like this, especially with the stuff about Charlie never really getting on with her dad.
I can't wait for Ruby to call Charlie 'mum.' I think that Charlie is really trying to make an effort to step into that role herself which can't be easy given the 16 years of pretending to be her sister, and she is coming across a little overbearing with the whole diabetes thing, but I can sort of see why. She can finally act in the capacity of Ruby's mum now everything is out in the open, and she's sort of trying to find her way and change as she goes along, stepping on Ruby's toes a little in the process.
I agree with the above poster who said that they'll probably get the Ruby calling her 'mum' scene with one of them in the hospital. I always thought that would be the way they'd do it. I mean, it's completely obvious but it's actually realistic. I think Ruby needs Charlie's lives to be in danger and at risk of losing her to put everything in perspective for her. Fair enough she's 16 and her life has been turned upside down. She needs time to get over that and comes to term with everything that's happened, but the prospect of losing Charlie will make her realise that she needs to accept the past and move on, giving Charlie the chance to be her mum and having that great relationship or continue to be uncertain around her and possibly lose her before they've really had a chance to connect as a mother/daughter relationship. She's already lost one mum and she's kind of been given a second chance with Charlie but she could lose her too if she's not careful.