I think because of the situation, you have to step up. I dont know the situation, or why she came to stay with you, but regardless of the reasons, by the fact that she IS staying w/you, you have a responsability to her, and her future. You accepted the responsibility for her when she came to stay w/you. Like it or not, dem are de facts. You gotta step up. You can be her friend, but still lay down the law. make a deal w/her, tell her as long as she only misses class if she is really sick, you will give her added freedoms, staying out a little later, yada yada. Some might say, just force her to do it, but I am a pragmatist, and since you arent her mom, just a sister, you have to make compromises to get the job done. But remember, she in your house, and 15, or 50 as long as she is in your pad, you make the rules.