You could blackmail her, and perhaps she'd cave. There is also the chance that she's a raving bitch, and go crying to your father about how you're trying to break them up before you can speak to him, and remove your credibility.

Get evidence and then go talk to your dad. Somehow I don't think that he'd care for you keeping this a secret while you use the situation to further your own desires. Without some sort of evidence (preferably a picture) your step-mom is free to pull the "Why does your son hate me so? Wha wha wha, I try so hard!" card.

This is what I observed when a friend was in a similar situation, at least. Your milage may vary.