What do you do about dishonest family?

Forgiveness and allowing someone to keep abusing you have nothing to do with each other.
My wife has forgiven her abusive father, she has nothing to do with him because he has not gotten help... it is simple.
Get a lock for your room... that too is simple. If your grandmother does not allow that, move.
She is as much of the problem as your sister while you are living there... it may actually improve your relationship.
It is not "cutting her out of your life", it is moving on with a healthy life for yourself.
Family are those who love and support you. I have many people whom I am related to that are not my family and many whom are my family that I am not related to... again, the two have nothing to do with each other.