Actually no, she doesn't take the full brunt of his attack, but sets it aside so it's broken. As in, she parried properly. Also, she was quick and light too at the time, so since his attack was broken, his momentum made him keep going with his cutting attack downwards, giving her at least a fraction of a second to counter with a quick slash at the neck. 

The fight only lasted a few seconds. It went with him charging with a diagonal cutting attack, her side-stepping and parrying it, then slashing at his neck while he was trying to lift his sword to attack. After that she moved back some steps to escape his sword range. Since his target at first was her father, and since he was dying, he charged at him then, not caring about anything else. So that made his back be open to her, thrusting her sword in it. Not long after, exsanguination. Sorry I didn't include this.

Eh, well that's just my logic. Alright then, I'll change her age to maybe 12?