Am I stepping over and beyond my bounds?

I have an 8 year old stepdaughter. She is very protective of dad and after 15 months of marriage, things have not settled much. When she thinks her dad and I spend too much time together (she says whenever he pays attention to me instead of her), something always gets broke or the dog gets hurt. I used to let my husband deal with her but I've finally had it. I made her sit in timeout for 2 hours to keep her from destroying anything else. Recently she got caught stealing lipstick from a little girl at school and then tried to lie her way out of it. My husband was just going to talk to her. I told him that he really needs to discipline her so we grounded her for a week from TV, Wii, and her bike. Not 2 days later, she told another whopper of a lie. I know it's going to take time. I just wanted opinions on whether I am overstepping my bounds as a stepmother. Any advice is welcome.