The twist is there, and it’s a strong one, but it does feel a little rushed – the problem is you’ve taken a serious issue and then instantly resolved it, missing out on emotional reaction and development in favour of swift revenge.
I still enjoyed it – the first 80% is gritty and hard-hitting, well-writen and engaging, and you can’t deny that finding out it’s her father adds something to it… But I still found the ending weak – to an extent it undermined what you’d built in the beginning and middle. MH :)
It is a hard challenge – your story was excellent for the most part, and the twist was shocking. Its a shame about the last line. Maybe you should add another sentence, like where she killed her father (in the same room?)
In fact, you could get rid of “she never saw her father…” and start a paragraph with, it was fifteen years later, in the same room. And add something about the father being old in your last sentence