Listening and Arguing

B, my husband's granddaughter is stressing me a lot as compared to my kids. She's getting way too comfortable with me that she don't hesitate to argue and disobey me. She continously give me headaches. Everytime she's in my nerves, I just close my eyes and try to understand her situation.She don't have a mom and haven't even seen her dad. She's separated with her only sister. So unfortunate! Even though so, she's using her situation to manipulate us. But I won't let her use me. I told her how I felt the moment she commit mistakes. She's apologetic but still keep on doing the same stuff. That's why I keep on changing my style in correcting her. There are times that I will ignore her and let her deal the consequences.For instance, she missed her reading for the day because  she would rather play, so on our tv time, instead of watching tv she has to read. Sometimes she don't listen when I told her to wear pants when she's riding a bike, instead she wore dress or skirt and when she got hurt, she can't whine about it. So today, I will not let her argue with me anymore. I will make her realize that I'm the parent. I gonna have to be consistent with her. May God help me and guide me too.