I'm totally against rude entitled teen girls, don't get me wrong, but you need to take the annoyance out of the equation if you want to state your expectation on this.
They probably had no idea it was rude if they call the shots in their homes or didn't think you'd care. It's not a universal rule that the only polite way to eat is to eat all at one table. Lots of "company" dinners have kids separate from adults etc. It's not like they showed up to the table and gave you attitude, they just asked your daughter to ask you if they could eat separately.
If you want "better behavior" going forward-as in behavior you find appropriate in your home-you need to tell your own daughter how to enforce it. Them: "Can we eat separately, we don't want to eat with the boys" Your daughter: "Actually the rules here are eating all together." Period. Done. I doubt any further drama would ensue and if it did they could never come over again.
Also, you sound pretty annoyed with the only child from the huge house, but in the end, she did come over to your house for babysitting, and they were fine, so again, you're offended everyone would rather go to her house, but no one meant to offend you. I GET it, I may feel the same way, but you have to let it go when it didn't amount to any offense. When I was little, I had a rich friend and I loved going over to her huge house too.
If you want the teen girls to dine with you in the future, just state it that way to your daughter and to them. "I'd love us to all have dinner together, I'm serving the whole group if you'd like to join us." And don't feel bad when you say no to their demands, ever.
The worst option is to "cave" and then feel angry or assume they meant to be rude. And if you know them well enough to know that they really are just rude girls, then maybe your step daughter needs less overnights and dinners with them. When I was 16 I had two jobs outside of school and hardly any social time.