Can the landlord evict my sister?

My sister is unemployed with a child to take care of. She is a single mother with no savings. She has moved into my dad’s apartment. It’s against the lease. But my sister is adamant. She says no one can evict her as she is a single mother. I am ready to find her an apartment. But she is not ready to leave the house. She has a poor credit score. She used to spend money recklessly and that is my sister couldn't pay her rents. Now she has no job. But she has changed her spending habits in the last 3 months. What should I do?