Thanks for your question. My name is XXXXX XXXXX I'd like to help you out.
I'm sorry to hear about what you're going through with your sister, and I can certainly understand why you would feel the need to cut the cord after all of this time. She is obviously doing something very passive aggressive right now in giving you the silent treatment in exchange for the distance that you've already created between the two of you.
The fact that you have continually tried to help her throughout her life is admirable, but considering the way that she continues to take advantage you, it does seem necessary to continue to distance yourself. Since you have admittedly been trained to take care of her, that is a tough mindset and mentality to get out of. It would probably also be much easier if this were a friend or a distant relative, but when something like this happens with your own sister, there is always a tendency to try and remain optimistic that the situation will improve and the relationship will get healthier.
That doesn't make you a wimp or mean that you aren't strong, and I do believe there is strength in your willingness to continue to extend your hand and try and help her. However, if you have reached a point where you know that this is no longer possible or that things will not improve, it probably is time to cut the cord. If that's the case, sometimes it can help you personally to do that if you were to explain why, either on the phone or by letter or email. When dealing with someone like this, it is likely that she won't respond well, so if you were to do something like that it would have to be more about you getting appropriate closure so that it is easier for you to cut the cord. Cutting the cord is very symbolic and would be the end of prominent relationship in your life, so even if it makes perfect sense, it is not going to be easy no matter how strong you are. It is most important that it's done in a way that will give you as much peace of mind as possible so that you can move forward with your life.
I definitely wish you the best with all of this, and if there's anything else I can do to help please let me know.