I don't think it's disloyal to your sister to discuss her in therapy. That's the safe place to talk about her, rather than discussing her with other family members. Just because she wasn't a good person as a kid doesn't mean she can't change - plenty of kids who are awful an change and can evolve into great adults. You still need to work through your childhood issues however, and that's where therapy comes in. Sometimes things can be worked out with family, sometimes not, that's where therapy comes in. If you come to terms with the past you may be able to engage with your sister based on who you both are now rather than who you were years ago. I see it as a positive step in your relationship, whichever way it ultimately ends up going.