How can I help a jealous 8-yo step daughter?

Well, it just sounds as if your step daughter is spoiled rotten. The way I was raised, was if you acted like that, you'd get the tar beat out of you. Im not saying hit her, but a child IS NOT abuse. No matter what people say. It is a form of punishment, older than the "sit in the corner". Even, though you may not be a religious person, the text's of religion say "If you spare the rod, you spoil the child." This child has apparently never had any actions taken against her, and to me, it sounds as if what she needs. Yet this is a touchy subject, especially since she's not your actual biological daughter.