Zoom said:
If you REALLY want to be mean, er proactive...give your roommates instructions to call the police on your sister for trespassing if she ever shows up again.
Family is irreplacable and, no matter what, they'll be involved in some form of your life for the rest of yours. It's heartbreaking to see your family in such turmoil. If only for your peace of mind, I'd like to suggest you seek a family counselor who could help you repair this rift with your family. Do you or your mom have a clergy person whom you could talk to? Many times having an impartial person to act as referee can help with severe family problems.
Even if you have written-off your family, counseling can at least help to create peace, if not love, among family members. It's one thing to have no contact, another to have family members actively seeking to create problems in your life. Counseling can't make people love each other but at least it may help you get a peace treaty between you and your family so that you can live your life without their harassment. At best, a good counselor can bring your family together and help heal wounds. At worst, your family may not cooperate but, if they don't, at least you have the moral peace of mind and satisfaction of knowing you did what you could to help repair things between you and it was they who rejected your suggestion to help patch things. Seeking a counselor can help you be happy and help you all move on with a happier life.
Knowing you tried to patch things, knowing you made an effort to resolve problems, even if it doesn't work, will be extremely important to you. Regret is a terrible phantom. Don't let it haunt you. If your family decides not to participate then that's their misfortune. You can't control their actions but you can control yours. Resolving these problems, one way or the other, will help you lead a better life.
I wish you luck!