Step parents - how do you deal with the new children and their dislike of you?

I've just moved in with my sister after her husband left - I'm the nanny now. I came bringing a harder level of discipline / tougher requirements on bahavior and the 8 yr old is bucking against it and I feel like an evil step-parent. She really gets under my skin. I've been there 8 months now and it isn't getting better. I find that sometimes I have difficulty not reverting to elementary tactics of bickering with her. Any advice on how to get this relationship working better?