I don't think you over reacted at all, I would have reacted the same way if I had been in your place. This is one of the reasons some here have said forever that when you see a girl out in public not to approach her, it's worse when Your in drab and at work by a thousand times.
I can understand the other girl trying to reach out though, who doesn't want to have a sister as a friend who understands everything that you may or may not be going through, someone to talk to, or just hang out with. She just made a bad judgement call when it came to time and place that's all. At least she showed up in drab, had she been dressed, things would have been a lot worse.