Platies can store sperm. She was probably either pregnant when you got her, or she got pregnant once you've had her. There is no way to tell who the father is.
And it's normal for the male to be chasing her, Mollies are somewhat aggressive fish, and pregnant fish secrete a hormone that Males find attracting. He'll be chasing her for a while, though make sure he doesn't stress her out too much.
Try all you can to make sure brothers and sisters don't breed. If it happens, it happens, but try not to let it happen. It will just make the fry weaker and more likely to have deformities.