Step daughter is a pathological liar...any advice?

I think there is definitely a problem. Kids do lie but what your explaining is that it is not just an offhanded lie, she is making up elaborate stories to go with them. My advice is that the first thing you should do is go to her father. Make sure he discusses everything with the mother first befre you do anything though as if they believe you are taking their daughters wellfare into your hands without consulting them it will cause unneedded friction. So first explain to him what is happening using examples and experiences but make sure you don't sound like you are trying to get her in trouble. Then tell him that you think it might be beneficial to take her to the gp just to make sure everything is alright, make it sound like it is just a checkup though because if you make it sound like you think she is crazy then he will just go on the defensive. Tell him that, as you say he just ignores it because he can't be botheres dealing with it tell him you will take her to the docter so it doesn't inconvenience him. The doctor will ask you to explan the problem and if you ask they will probably let you be alone with the doctor for this bit (Which I would also reccomend as the child might feel they are being 'told on' or be embarresses or hurt) and then they will probably call her back in for a couple of question and then make a decision on that on what the next move should be. I can tell you care a lot about her so getting help and making sure everything is alright with her is the best option and if you go about it the right way things shouldn't be too difficult :)