You may not ever get a response from the parents, even if you send another note to the girls. Who knows if the invites ever made it home from the classroom. I think if you want to know what's going on you need to contact the parents yourself- are they in the phone book or is there a school directory you could get, or maybe just the school office could give you the phone numbers?
As for the party itself, you potentially have 4 girls coming to the party. Renting the birthday room at the theater sounds like a lot for 4 guests. Why not go to the movie with whomever shows, then take them to dinner or just to dessert at a restaurant to open presents. Then you're set with a plan regardless of how many girls show up.
In the future, maybe you need to give your step-daughter a concrete deadline for when she has to tell you what she wants- show her the day on the calendar and remind her that the day is approaching. Tell her that if she doesn't have an idea by that day, you'll come up with something. Also, she may need some help coming up with an idea. Maybe it would help her (and you in the planning) to sit down one day and think about it together. You could make it a whole activity getting her involved in the planning of the party. Or, if she tells you two weeks before her birthday that she wants something that requires more planning, tell her you can do that but it can't be on her birthday because you need more time to prepare and tell her how long after her birthday she'll have to wait.