I can see both sides. On the one hand I have a friend kinda like that but I'm not around her all the time so I don't really know, maybe something has developed. On the other side your sister may have been picking on you all that time as a way to conceal her own shame/issues around her own anxiety. Either way, the best lesson I've ever learned is that we have absolutely no say or control over what others do/say/think/feel ... only how we react to it. If she is being a snot and trying to get under your skin then the best thing you can do is not let her and if she really does have a problem she probably needs your compassion and knowledge on the subject.
\"As soon as you trust yourself,you will know how to live.\"
Johann Wolfgang von Goethe
\"Do not anticipate trouble, or worry about what may never happen. Keep in the sunlight.\"
Benjamin Franklin