My 12 year old daughter has started acting like a brat, it's driving me insane.?

My 12 year old daughter is driving me crazy. She used be a great kid, but wow, I could cry now. She's my step-daughter and she acts likes she brainless. She refuses to do her chores then tells me how unfair life is because we punish her (she didn't do her because "I didn't want to" so we gave her half an hour early bedtime and told if she's not doing chores she's not getting allowance, so we're not being unreasonable). She has been become so selfish. She told me today that people are suppose to take of themselves first then look after the family... I was heartbroken, I have no idea where she'd that attitude from. That's the opposite of how her father and I are. Today I told her I would like her to have a skirt or dress in her wardrobe in case we go somewhere that requires one and she sulked to the point that she almost cried, because I asked her to come buy a skirt! It's driving me insane, she used to be so sweet and now she's a spoiled, selfish brat who stories are just short of lying. What do I do? I am going insane.