To the person who screwed over my sister, let me just remind you that karma is a bitch.
I understand that you have a complicated relationship with your father, that every boyfriend you've had in the past few years has rightly decided that you are a nutter and dumped your sorry ass/called the police on you, and that you've had difficulty keeping the past three jobs - I get it that you have "issues". However that did not give you a free pass to f*ck over my sister.
You stink. You stink like a bag of piss.
The right thing to do would have been to tell her that you were working with someone else and to take her to a nice dinner and say thank you for your help.
But you did the wrong thing. Which frankly doesn't surprise anyone. Because you seem to excel at doing the wrong thing every single time.
Good luck to you. As I mentioned, karma is a bitch. Or perhaps, who needs karma when they have the life you live.