Our dd is 13.5 months older than our (identical) twin sons. They were born at 36 weeks, but had very few preemie issues beyond being small and gaining slowly, initially.
Both our guys started talking at around 14-15 months. They didn't talk much until they were about 22 months, though--just the occasional word and lots of grabbing and pointing. They are very articulate 4yos now.
I never really noticed our dd talking for them--she was usually more concerned with making sure she got what she wanted, much like your dd. Perhaps someone whose older child was a bit older than my dd has had a different experience.
Our little guy, on the other hand, is 16.5mo, and doesn't really talk at all. I'm not concerned, though--he is very effective at getting what he wants! He does say mama, dada, dis (thanks) and ball...and has lots of noises he makes until he gets what he wants. He jabbers all day long, though--it's just not in English.
Honestly, at 17 months I wouldn't be concerned, yet, as long as they are interacting with each other, their older sister, you and your dh, and the world at large. There is a possibility that there is a delay, but 17 months is still well within the range of normal for speech to be just beginning. Were they full term? Are you adjusting for any normal
delay because of being preemies, or because of any trauma/medical issues at birth?
I hope this helps....Update us after the appt later this week.