There is not much that you can do in this situation except limit the time you spend talking to her or stop talking to her completely. As you are beginning to recognize, she is likely not going to change. Based on how you describe your calls with her she is not interested in your advice and does not seem to want to listen to your suggestions. You cannot force her into doing what you think she should and you cannot handle these situations for her. Therefore you are only left with a few limited options, namely boundaries and limitations.
You can set up boundaries and limitations by literally decreasing your calls or restricting the time you spend with her on the phone. This is one way. Another way to set up boundaries with her, when you are on the phone, is say things like “we already discussed this and I already told you how to handle it. I do not wish to continue discussing this.” Or saying “I will no longer discuss these issues with you since we have already been over them.”
The idea is to be firm and assertive with her about how you want the call to go. Apparently others have set up boundaries and limitations with her and maybe others have simply told her to stop calling. Perhaps she is calling you because you are one of the only few that she can still call. If you want to get off of the phone, then you need to simply say that you have to go. She might not like this new way of dealing with her but if you do not do this, you will continue the nature of your present phone conversations.
Also, try not to feel guilty about wanting to get off the phone with her. From your letter, it seems like you do try to help her and advise her and she apparently completely ignores you. It is good and kind to try to help her and you should continue to do this but stop when it becomes apparent that your best efforts are simply not working. You need to recognize when you reach the point where you simply can do nothing else which will be lead to a positive outcome.
The only way to change this situation is to change your behavior towards her, take control, be firm, define how the conversation will go and limit the time you talk to her. Do this without feeling guilty. There is nothing to feel guilty about. You can still love your sister without being held hostage to her calls and negativity.
The simple question becomes: are your phones calls helping her? In cases like this, the answer is almost always—no. The nature of the mental health system in America is such that if she doesn’t want help or doesn’t recognize that she needs help, unless she is homicidal or suicidal, she can not be forced to get help. Be proud that you have tried and continue letting her know how much you love her. Best of luck… to the both of you.