First of all- if you give a shit about your father then forget about blackmail... it's only going to fuck things up more.

Give whatever evidence you have to your dad.
Let him decide what to do.
DO NOT let her know whats up.

Think of this like a job. In most decent jobs if they have to let a person go (for any reason) then they fire that person and give them at least two weeks pay. They don't tell a person "ur fired bitch, be out of here in two weeks." The reason being is because that person can really fuck some shit up in two weeks time. If you let your dad know whats up, then he'll have the time he needs to prepare (cancel the credit cards, move some cash around from bank accounts... you see where i'm getting at). If she knows shit is about to go down she could really fuck shit up for you guys.