Based on what you said, it sounds like you are describing traits of borderline personality disorder or the actual disorder itself.
With borderline disorder, one of the main symptoms is instability in relationships, which is what you are experiencing as well as sudden changes in mood. They also experience emotional disturbances, inability to control anger, sometimes threaten suicide, impulsive behavior and irrational demands.
You are not alone in your frustration in dealing with someone like this. Often, since the person with the problem lacks insight and is unwilling to seek out help, it is left to the family or friends to find a way to deal with the person's issues. You can first begin to protect yourself by educating yourself on what you are dealing with. Talk with a therapist to help you cope with the situation. Seek out the help of other relatives if possible. Instead of accepting what this person is subjecting you to, reflect back the feelings and put them back on the person. Leave the situation if they start to rage and get out of control. Tell them calmly you will be back later when they are calm. Keep in mind that dealing with this person can take a toll on you so you need to, first and foremost, take care of yourself.
There is a book that can help you cope called "The Essential Family Guide to Borderline Personality Disorder: New Tools and Techniques to Stop Walking on Eggshells" by Randi Kreger, co-author of the bestselling Stop Walking on Eggshells and Stop Walking on Eggshells Workbook. Also, a book called "The Borderline Personality Disorder Survival Guide" by Alex Chapman and Kim Gratz would be helpful as well. Both are available on Amazon.com or your local library may be able to get them for you.
I hope this has helped you,