Am I ready for this?

You may not know, because I probably don't type about them much, that my husband has two kids from a previous relationship. They are 14 and 12. When Chad and I first got together the kids came over regularly and we blended pretty easily. Not long after Ashley was born the problems started. Now I know that they were just repeating a lot stuff that they were hearing from home but some of it was very brutal. After their mother had a failed marriage, she decided she was going to do whatever it took for her and Chad to get back together. All the things that happened during this time ended up straining our relationship and Chad's relationship with his kids. They eventually stopped wanting to come over and we stopped making them, it was just too hard on all of us. We've seen them on and off over the last couple of years. Not seeing them has bothered Ashley more than anyone. This year she happened to be in class with a child that lives with Chad's kids. A couple of arguments, a parent teacher conference and a phone call later, Chad's kids were coming over every other weekend. The last 3 months have been great! It was like they had been coming over all along. The kids all got along. I stepped back a little to let them get reacquainted with Chad. It's really been nice until this weekend. It's Chad's daughter. She is 12. She thinks she's 20 something. She does what she wants, talks like she's grown with no respect for adults or other people that are around her. That may all be being 12. Here is what I had a problem with this weekend. After we came home from them mall Friday night (where she wanted us to just drop her off) she got on her my space (which she is not supposed to be on but her mother won't delete it) and told her friend that she was at the mall with her ex boyfriend and they ended up getting back together and she was spending the night at his house. Now first of all, I can't stand liars. Second of all, she completely disregarded has father's rules that she was not to be on her my space. Third, the things that she is writing are going to give her a reputation that she may not be able to get rid of. Now here is my delima, I am raising my kids to be good people, to behave a certain way, to have respect for themselves and other people. I have not raised Chad's kids. I cannot make them behave a certain way. I don't want to parent them. I want to be an example for them. I want them to look at Chad and me and know how people are supposed to treat each other. I DO NOT want my daughter to look at Chad's daughter and want to be like her. I find that the more Chad's kids are over the more my kids try to get by with acting the way they do. Chad's son has been wonderful. I don't always expect him to be this way, he is after all a teenager. He seems to motivate my son to help around the house and to be more active. I know Chad loves that has kids are coming over, so am I. I just don't know if I am ready for what more may be coming.