I certainly have some experience here and asked for the same type of advice many months ago.
#1. Find a therapist for yourself (You have created this situation by allowing her to control you through her words and actions)
#2. Cut her off financially on non-essentials. (If she is home for the summer and does not have a job, she does not need her car....take the keys....cut off her "fun" funds, but continue with tuition, etc) Her behavior will get so much worse when she realizes that she is lossing control, but control is what this about. Disrespectful behavior is not normal.
#3. She should go to therapy too, but, and this is a huge but, when kids get like this, sometimes they will never see the light. Therapy will be labeled as "dumb" the therapist will be an a$$ (because....gasp...he or she will want to set limits and end the reign of terror)
#4. This situation is difficult, horrible, name the word. I was very reluctent to grow a backbone with the child (now 22) in my life. He has not changed (actually got much worse when I started to change...but has leveled off), probably never will, but I have. When he lays into me (we both work in a family business...sigh) I simply walk away. He will literally throw a fit. Makes him look crazy, but I will not be his punching bag. I have now dealt with a few family situations where if he acted half way human he would have been a part of, but I refuse to have him in the house if can't promise his father or siblings that he will behave. (this means missing milestone birthdays and showers, etc) And I never thought I would say this, but I can live with these choices. Is it painful that he can't be there for our family?...yes. Is there peace?...yes. Everything has its price. My price for peace and respect is a small piece of my heart is torn, but I also have younger children and it is my duty to them to show them that respect and caring is not an optionial.
#5. If you go to therapy and you get the right the therapist, be prepared for some serious reality checks. My therapist wanted to know what about the situation I thought was ok to let it get this far. My son's behavior even made him angry. In a way empowering, in a way, hurtful. If you can't be honest about just how horrible the behavior is (and I believe the way you describe your situation, the behavior is bad or you would not be here) make the change. It will change your life.