Oh my gosh!! You got a mother from hell on your hands....Once you start carting her kids around she will expect it all the time. I only say that because she was brazen enough to mention the twin as if "you have one over you have to take the other". UGH! That is not fair to you daughter since she expected it to just be her and girl #1. My youngest daughter was friends with a little girl (really a nice kid) but her mother was a nightmare. She too did not drive so she calls everyone in town for rides. She even went so far to make a play date for her daughter with a friend of mine's daughter and then "suggested" to my friend that when the playdate was over she should take her to the grocery store to do her shopping for the week. Seriously!
I think you should definitely nip this in the bud now before it gets worse. Obviously since you already agreed to take the twin then I would follow through with that. Then after, talk to your daughter about her relationship with girl #2. If she is not someone that your daughter is that close with she and you are not obligated to include her on playdates. If it comes up again, it can be said, your daughter can only have one friend over at a time; you have a houseful yourself and enough to handle.
I also know another mother who has two girls (ages 9 and 7) and when one of the girls is invited to a birthday party, the mother comes out and says that the other girl must come to the party and they are a "package deal". To me I think that is totally rude. I have avoided this by luckily, my daughter not inviting her (by her own choice)! But there are people out there that just have no sense of manners and think nothing of inconveniencing other people.
Good luck and let us know how this plays out!