I have a brother who is similar. I try my best to get along with him but to no avail. I have to have my mother use his phone to skype me so I talk to him but there is no connection. He is the most abusive and insensitive individual in my life now and truthfully, I wish I can do the same thing you are doing to your sister. I know how you feel and understand the reasons you are cutting her off. So, don't feel bad about it. You can choose your friends at times, but not your family.