A group of friends and I had made plans to go out to dinner for NY Eve's, and it was a large group, reserved table kind of deal. I invited another friend of mine who didn't know any of these other people as she didn't have any plans.
You know what they say, No good deed goes unpunished.
Recently an old friend turned 40 (we'll call her "the mom"). Her DD called up a mutual friend and told friend that she wanted to throw a party for her mom and wanted us there but would call back later with the details. So we waited and waited and nothing ever came of it.
Later I decided since nothing was happening, and it was Thanksgiving, I'd invite the mom and DD, along with the other friend over for dinner, and give the mom a little gift at that time. Invitations we're issued and then the day before Thanksgiving the mom calls up and asks, since it was her Birthday, if she could invite her DD's friend along. I hadn't said anything about her birthday yet, and was quite surprised at the audacity, but put on my best E-hellion voice and told her that "just wouldn't be possible." She whined and wheedled but I stuck to my guns (it really wouldn't be possible - my apartment is tiny, and she knows it).
They came and didn't bring the extra guest but behaved terribly all night. The following week the mom called our mutual friend to complain about my not allowing her DD's friend to join us. Friend expalined that DD had actually invited US, but never followed through, and that if DD wanted to invite people to a party she should have thrown it herself. We would have been more than happy to help, and as it was, I went out of my way to provide a vegan thanksgiving dinner for them. The mom said she'd talk to her DD, but I haven't heard a peep out of either of them since that night (and that's October for all you non-Canadians). I've learned my lesson!
At least in your case you only ended up with one embarrassed person, rather than the two you would have had if her brother had shown up.