By not enforcing the rules, you are enabling her to not have to grow up and be responsible for herself.
By her not following the rules, she's clearly a person who has no respect for other people, family or not, and is using you and enjoying the benifits of continuing to be childish.
SO, I'd say give her ONE more chance. Next time she's disrespects the house rules you've laid down, she's out. If she has a job she can afford her own place. Let her trash her own place, not your place... Your home is YOUR home. Being taken advantage of and disrespected in your own home... Intollerable...
By her not following the rules, she's clearly a person who has no respect for other people, family or not, and is using you and enjoying the benifits of continuing to be childish.
SO, I'd say give her ONE more chance. Next time she's disrespects the house rules you've laid down, she's out. If she has a job she can afford her own place. Let her trash her own place, not your place... Your home is YOUR home. Being taken advantage of and disrespected in your own home... Intollerable...