Hey there K. - I have been through a similar situation. My husband and I married when his daughter was 13, she's now 17 about to turn 18. We never planned to have any children of our own & had told his daughter this... however, we got a surprise & now have an incredibly beautiful daughter, 10 weeks old now. (She was not planned, but is incredibly well loved by us both.)
I should point out that the relationship my step-daughter & husband have isn't the best due to her mother's influence, so we were dreading telling her! We phoned my step-daughter after the 12th week & my husband was the one who told her as we live in a different country to her. Thankfully she was very receptive to the whole thing.
I'd suggest, though, that you both tell your step-daughter in person. If we'd been able to, we would've told her in person.
We included his daughter by actually using the first name she suggested should we have a girl. This made her feel included and she has seemed very keen on knowing about this baby of ours ever since.
We did also tell her that it was a surprise and we hadn't planned it.
I wish you all the best with the pregnancy and telling your step-daughter. She's been through this once before & while she was upset she has survived it. Include her as much as you can, share your excitement with her and she just might come around.
Edited to add: I like the fact that you had discussed with her over the last 2 summers the possibility & how she felt about a sibling. At least she knows there is a possibility that this might happen! And you and your husband had the opportunity to ask her how she felt at the time.