Are you sure that she really doen't believe that you don't love her? Or is this a case where she's in an argument, you're trying to be the negotiator and she's not happy that you are not siding with her completely? It was something that was said in the heat of the moment and has more to do with her not getting her way? Teenagers do that. Even the best of them.
I would suggest that you take what happened last night out of your memory for the time being and look back and the entire context of your relationship with your niece. The memories that you have there, whether or not she has been receptive to your love and attention, and whether it was reciprocated by her, should tell you more about what she believes in her heart to be true than what was said last night.
It's completely understandable that she may feel put off by your stepdaughter moving in and going to school with her. Her sacred territory is being encroached up, after all, by foreign invader (just trying to channel your niece's mindset here but may not be doing it very well). And she very well may have issues about what her parents did or didn't do that she may need to work out. If you feel that is the case, then contact a school counselor to see if there is a counseling service in your area (she may not feel comfortable going to see a counselor at her school) that offers free or sliding scale therapy for her.
I hope this all works out for you and the girls. Have heart. Sometimes teenage girls say things that they don't really mean when we are really, really pissed and frustrated. Us grown up girls do that also. Take care.