Why not treat her like she's acting. Lying is something kids do to get attention - positive or negative. So why not take away ALL of her privileges until you can trust her. Tell her that she needs to earn you and her stepmom's trust. And that you understand that she's learning how to be a "big kid" and that when she starts acting that way consistently, then you'll reward her behavior.
I believe in cause and effect. And I also believe in earning privileged like TV time, computer time, phone time and outing with friends. Everybody has to learn that their behavior affects not only how people perceive them, but also trust in future situations.
Maybe you need to have a sit down chat with your daughter about how important TRUST is between you and her. You need to trust what she says and vice versa. That includes keeping your word - if you promise to do something, then you do it - NOTHING can override your promise. Ask her how she would feel if you started lying to her. Telling her you'd take her somewhere, like out for dinner or getting her a new outfit, and then went back on your word. Maybe explaining that her little lies are a breach of trust and have consequences like losing her privileges will wake her up.
Then when she proves to you that she is trustworthy again, she can earn one privilege back at a time. If she chooses to lie again, she loses it again.
I wouldn't focus on "how big" of a lie it is. Instead I'd focus on the trust issue.
And I'd also treat lying to you and her stepmom the same. A lie is a lie - period. There should be consequences whenever she lies.
If all of her privileged are taken away, maybe then she'll start to understand that she put herself in this situation. And that losing your trust means being stuck in the house, until she can straighten up.