A person has borderline personality disorder if they meet a certain number of clinical criteria including many of the symptoms that your sister displays. However, only a trained clinician who met with your sister in person would know for sure if she suffers with this disorder, and like you mentioned, it is unlikely that she would attend such an evaluation.
Even if your sister would attend an evaluation and seek treatment, it may take a long time for her to change. Even the best treatments for borderline personality disorder can take months to years to help a person get a handle on their symptoms and alter their long-standing pattern of behavior. The bottom line is that even if she is successful at changing her behavior, it could take a long time for you to notice a marked improvement.
Because of this my suggestion is that you will likely need to learn better ways to deal with your sister. This means that you will have to change your behavior towards her. For instance, you mentioned that she only seems to want you as a sounding board to which you provide for her at her request. This is a big mistake and is particularly unhealthy for you and an incorrect way to deal with her. It sounds like you have to “walk on eggshells” whenever she is around. You do this because you do not want to lose her but the problem is that you are essentially letting her abuse you or act inappropriately towards you. She in essence is getting away with acting a certain way towards you, in an abusive and inappropriate way because she can get away with it from you.
It would be better for you to define when you are going to be there for her and it should not be when she demands that you be there. You can be there for her when you are available — on your time — and when she can learn to talk and treat with you with more respect and not make unreasonable demands, yell or scream or act out inappropriately. You should not have to “walk on eggshells” around your sister and the way to stop this is to better define how she can interact with you. If you do this successfully and in time, you will change the dynamic of your relationship with her. You will not cure her (if she has an illness such as borderline personality disorder) but you will have a better and potentially more stable relationship with your sister.
Finally, let me warn you that if you begin to change the way you interact with your sister (i.e. you no longer be her sounding board whenever she demands) this will anger her at first. To be successful in permanently changing the way you interact with her, you will need to let her be angry with you. In time, her anger will decrease and she will ultimately learn this new way of dealing with you on your terms.
I hope this helps answer your question or gives you at least a little insight on how to deal with difficult and unreasonable people. Please write back if you have any more questions or need clarification. Take care.
This article has been updated from the original version, which was originally published here on November 7, 2007.