A friend of mine getting a graduate degree in psychology also has studied the illness and immediately knew that's what my sister was suffering from. Now that I have read more about it, we are almost 100% sure that this is what she has. 

She is 31 years old, and of course, very challenging. She rages all the time and would take the idea of getting help very offensively. (as she has taken it in the past). 

What suggestions do you have for such a person? I fear each day that we are going to lose her because she is so unstable. Please help. Thank you.

A. Thanks for your question. Unfortunately, there's little one can do to convince someone else to seek formal help or treatment. The first step is for her to acknowledge that there is, indeed, a problem, and that she needs help with it. Even if you could convince her to see a professional for an evaluation - she would likely resist further treatment and/or not follow through with any recommendations made until this first step is taken. I would suggest gently speaking to her about it over the course of the next few months. If she is in a dangerous or self-destructive situation, you may need to reach out to other family members to organize a "family intervention", in which you would meet together, share your joint concerns with her, and ask her to seek help.