Some quick background.
I am the second of five children in my family. I live with my older sister in a city located at 88 miles from our hometown. I'm 27 years old and my sister is 29. Meanwhile the rest of my family live in my hometown.
My youngest sister (20) had been acting the past months in a way that is already unbearable for the rest of my family. She is mean to everyone and doesn't like to do chores, recently quit her job and sleeps almost all day. Two days ago, she had a huge fight with my mom and left the house without giving notice of where was she going to go. She was "missing" for about a day and a half, didn't answer calls and had all of us worried for her.
She finally called me and told me she was staying in a hotel room and eating junk food (she has the money of her severance payment) and also told me she was definitely not going to return home. She asked me to let her live in my flat (the one I share with my sister) and if I said no, she would do anything else but return home. I said yes, but I know she plans now to settle in this city, find a job, etc., the thing is I don't really want to live with her permanently altought she is my sister because our personalities don't have a good match and I don't entirely trust her. I just want to help her temporarily and I want to state that to her really clearly to avoid misunderstandings.
What is the best way to make clear to my sister that I won't let her to live forever in my house?