MY Issues with Co-Parenting

This is the first day my daughter has been back from spending the long weekend with her dad, his girlfriend and her 9 yr old daughter. Usually, after a weekend with them she is returned to me exhausted and cranky from being dragged around, taken off of her schedule and long late night road trips to and from West Palm Beach. A 4-5 hour drive depending on traffic and speed. These are all issues that I am not only well aware of, I am 100% accepting of them. It's hard but normal and I can deal with it. What I'm having issue with is NOT normal, at least in the scope of my limited experience.

Scott and I try very hard to be consistent with our discipline and expectations...at least we are every time we talk about it. I am always up front and honest about how I feel things should be done and I am open to trying new things. I'm open to his thoughts and ideas. Hell, I believe we all could use some "outside" help at times. Different perspectives can be so very helpful when dealing with children. I get that even though we have open communication, there will always be some differences in how we interact with her. I feel certain things are unacceptable and he doesn't. It goes both ways. I'm not the only parent out there dealing with this and I guess I need some help.

My biggest problem so far has been returning attitude. His girlfriend has a 9 yr old daughter with more than her fair share of sass. I think overall she's a great kid. Smart and funny. I think there's a great deal Chloe could learn from her. And she is learning from her. The good and the bad. She always come back to me with a 9 yr olds attitude. A smart mouth (smarter than usual) and body language I'm not prepared to deal with yet. She treats me like I'm beneath her, talks down to me and has an endless supply of smart ass retorts for everything I say. I ask her to do something and she grunts at me. When I reprimand her for it and send her on her way, she mutters under her breath. She's being completely disrespectful to her brother. Using her older age and larger size to intimidate him. I understand siblings will fight and she will use those things regardless but this is in a way.....it's purely mean. I cannot understand that nor will I accept it as par for the course. Treating people like shit is NEVER okay in my house. NEVER. I teach equality and understanding. Compassion and love. Not ever an "I'm better than you" attitude. It makes me angry that I have to deal with this kind of reprogramming every time she comes back. It takes me weeks to get her back. It flabbergasts me that it only takes a few short days to undo everything I've worked so hard for 4 yrs to instill in her. She's seriously like 2 different kids. If I didn't witness it first hand, I probably wouldn't believe it.

So, here I am. The first day having her back and I am stresses beyond belief. I missed her so much and all I want to do is snuggle her and love her but she's making it so hard.

Is this normal? Do you think it's them? Me? Just the natural unbablancing of having two families? Two lives? I need some perspective here. I also need to talk to Scott about this but before then I need to know what should be expected in this situation. If I am making a mountain out of a mole hill. It certainly doesn't feel like a mole hill to me.....