When to tell step-child about new baby...

My step-daughter is coming to stay with us for two weeks starting Friday then again for another two-weeks at the end of July. I'm pregnant with our first child (step-child has a younger brother by birth mom). Anyways my question is when should we tell her?

She is a daddy's girl and being that she only sees him every other weekend I'm afraid she is not going to take the news well. That and I'm worried about telling her before the 12 week mark because if something happens then we have to explain that to her. Plus once she knows then her mom will know which means everybody will know.

Originally we wanted to take her to the 8-week ultrasound with us but I'm not sure if that's a good idea or not. My husband wants to tell her right away but I'm worried...

Any advice would be appreciated.