Perhaps she demands more I have twins in my Family and one is definatly more demanding than the other, do you treat her the same way you do the other Twin as sometimes they can feel left out if one gets more attention. Focus on her positive side and praise her when you see her doing something right.
My daughter tells me what to do e.g "mum, i want to have a party, I want you to buy lollies" etc I want my friends to come over here, she says "I want I want I want" all the time. Like , I say " let go out to shopping, fun..or let's go out for a night cool walk" I am always positive, she just cries and yells "no dont tell me what to do, I am not coming!" She hurts my feelings. Her twin sister is lovely,very a beauitful person, she listens and accepts. She is also getting fed up with her sister's behaviour problems, too.
.It may be a good idea to ask specific questions of your daughter regarding how her older step siblings give her a hard' time, do they say mean things to her ,ask her to explain what she means by hard time ,perhaps give her a time frame for her shower and tell her she has a while to do it or no TV or PC or anything she values doing, and stick to it, ket the shower be in her control and if she doesnt do it no favorite show on TV or outing . She sounds upset by something ,it is obvious your EX is going to say its not true what else would he say so find out what it is they do to upset her. When you say you refuse to do things she tells you to do, what kind of thingsand if they are requests would it hurt to let her have her own way? Is the other twin the same and does she say the boys are mean aswell what is her input here?