Hope,
I feel your pain. I have been dealing with the same situation with my mother/stepfather and my younger sister. She has stolen from them numerous times and my mother, older sister and I agree they should press charges but my stepfather doesn't want her to have a "record". I fear deep down his enabling is going to kill her. He just won't listen. He bought her a car, provides her a place to stay and gives her money while she sits on her butt not doing anything.
But you have to let it go. There isn't anything you can do to make your parents learn. They will have to get tired of it on their own. And your brother isn't going to quit until he hits rock bottom and wants to.
I know, You feel so helpless. This has been going on for over 5years in our family and I still broke down and cried for her last night. I fear I will never get my sister back. I don't want to completely give up but until she is ready we are just spinning our wheels. And you have to take care of yourself. I have learned that I had to distance myself from them and the situation. To be honest, I don't even want to be around my stepfather at Christmas. I have lost respect for him. It is probably going to ruin their 25+ year marriage yet because of her. But she doesn't care, she only cares about getting high. Just sad.
Hugs. I wish I had better advice or could give you some hope. That is what I hang onto. Somewhere down deep she will find that strength to overcome this.