Is it possible to treat your step kids the same way you'd treat your own?

I'm pregnant with my first baby, and I have a stepdaughter. I just get so frustrated with her lying! Her lies go right over her dads head, but I can sniff out a lie pretty easily. He is just oblivious (bless his heart). I want to point it out, but always feel like I'm going to come off as "picking on her". 
Background: she is 12 years old. Her mom lives out of state and sucks as a parent. My husband was a single father for a while. She hasn't seen her mother in about two years, plus she rarely calls. I don't want to treat her like she is fragile, but I also know it could be a fragile situation at some point. 

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