It's obviously been a couple of years since the attack that caused the lost of her leg. I wasn't accounting for the twins' age and just thinking that she would obviously be old enough to be at Beacon since Grey and Sylvia hadn't been married yet when she was three. The assassins' target was the girl for some reason, meaning that she was important somehow plot-wise. And the fact that she had her aura unlocked early, meaning Grimm would be more attracted to her, meaning that it was highly probable that she would train as a Huntress to fight the Grimm and the Beacon seems to be at least the most prestigious, if not the only college in Vale for Hunters. 
That's why I guessed her.