Just a thought. You can forgive someone and still keep them out of your life because they are toxic. Forgiveness in my opinion is a state of mind that let's you let go of the offense(s) and move on with your life. I don't believe it entitles you in anyway to put up with such disrespect from your sister. It is sad your parents couldn't admit the bullying that they had to have seen when the two of you were young and put a stop to it then. You will need to forgive them too, for not being there for you when you were growing up. I think when you do get in therapy you will need to work on forgivenss for them and your sister but for your sake not their's. You will have to just let your parents own their own feelings about the matter. You need to take care of you. Good luck