What question would you like feedback on?
How do I explain why it's not appropriate to sleep at a boyfriends house, where she wil l understand.
What is your reason? She's an adult and is most likely sexually active.
It is one thing for her to respect you as a mother and to respect your expectations. She's trying to fit the role of an adult and will do as she pleases.
The friction most likely arises from the fact that she's living at home. If she wants to do what she wants, she can then find a way to leave home and either live on campus again, with her boyfriend, a roommate/friend etc. She is not respecting your rules because she may feel a sense of entitlement (after all, you and her step dad are helping her by taking her to live at home (for whatever reason she's not living on campus)
The problem also lies in this that you nor her step dad or anyone else can make her do anything. Then, you've got to look at some sort of consequences- asking her to leave, if she does not have a job to get one, etc. She is relying on you for room and board at this point. Talking to her or reasoning with her is beyond what is going to be helpful. She needs her behavior to be met with the appropriate outcome. She acts independently and adult like, then she has to deal with what that sort of life offers. At the age of 19 she is not going to be flexible in looking at changing her behavior (that is causing you the stress) She will be looking at ways to satisfy her own needs/wants and for ways to do that. Hence the manipulative and dishonest behavior.
You and her step dad need your peace of mind. Try to think about ways to consequent the behavior or talk about what are reasonable expectations of a 19 year old. Are you reasonable in assigning her a curfew at 1 am and why the curfew? What do you think she would get out of it and what would you be getting out of it. Is she coming home disturbing you with being noisy, are her grades suffering etc. Examine the level of enabling that may have taken place so far and think of how to change what you and her step dad were doing. You cannot do the same things and expect a different behavior on her end. If she is living at your home, she needs to contribute monetarily or in any other way to the home environment.