Hi, Mike. That definitely sounds like a sticky situation. I believe that she may very well be a destructive person, out to take advantage of the man you are dating, and drive a wedge between you and him. I'm reminded of a girl in my office, who also had a child as a teenager. It's definitely not the case with every young mother, but she carried around lot of resentment. I felt she justified manipulating and backstabbing her coworkers because, in her mind, they had it easier than she did. After she and I had an argument, she actually lied to the boss, saying I kept harrassing the next day when I hadn't even spoken to her. I was afraid I was going to be transferred. Luckily, nothing came of it.
Eventually, I had to learn to tolerate her. I was working with her eight hours a day, after all, in an area where there were only four of us. She ended up quitting abruptly, moving back in with her parents in another state to escape her abusive boyfriend. I'm sure my boss didn't take her allegations about me seriously after that. She just saw her as troubled.
My instinct is, this situation might be analagous to yours. You just have to deal with her. Her father, filled with feelings of guilt, shock and warmth for his flesh and blood, won't see her clearly. Trying to enlighten him will probably backfire. If she's as devious as you suggest she is, her true colors will show in time. The truth always outs. I would think, at the very least, she will fade into the background once the excitement of meeting her has passed. The only question is whether the man you are dating is worth the stress and the wait. Only you can answer that. I hope everything works out for the best.