You asked:

"So do I back out from this deal?"

Yes you do. If they insist on knowing why: "I don’t want to live with my sister. She is toxic. It would create much more stress for me. I don’t want to be ungrateful but I also know there’s no way I can live with my sister. I can’t tolerate the drugs and alcohol and strange men. I just can’t."

End of discussion. If they try to badger you, say good bye and hang up the phone or walk out the door (whichever applies).

You will feel better for doing this.