Sick of drama

So I live in a house with my partner, 4 kids, 5 animals and a 18 year old. My 18 year old Step daughter thinks it is okay to treat us however she chooses (usually badly), and not do chores, or help out in any way. She is still in high school, so there is no option of helping her to move out on her own at this time
Sometimes I think she thinks she is a fairy princess and we are her slaves. It is our duty to provide food, shelter, electricity, heat, air conditioning, cable and internet for her pleasure. We are to do all of the household chores for her, and let her come and go as she pleases. And because we are such loyal slaves, she will occasionally bestow a smile or a than you upon us which we are to eagerly and gratefully accept, much like a starving dog accepts a bone.
She will usually speak to her mother in a snotty surly tone, but every now and then she will be kind and there is a glimpse of that sweet little girl with the sparkling eyes, quick laugh and happy smile. Those glimpses are few and far between these days, and we grow weary waiting for them.
People say, "oh, that is just how teenagers are". I refuse to accept that. I have three other kids that live in my house. They don't act as if the world owes them something. They do chores and happily participate in our family.

So the bottom line is I have to wait until July 2011 to help her move out on her own, and then my partner will spend months holding her daughter's hand as she navigates the world on adult terms. We try to teach her now while she still lives in our home, but she refuses to learn anything. We are stupid, and don't know anything. so we have nothing of value to teach her.

I suppose this too shall come to be a distant memory, but while living with it, life is difficult at best. I take comfort in the fact that we are not the only parent that have ever had to deal with issues like this, and we will survive.