well, if she did not let you know while starting relationship with you then she lied to you. she has her husband a baby and still wanted relationship with other guy means she is not a committed person. i dont know why she did that to her husband and baby, but that is very wrong. you can tell her that you started relationship with her thinking that she is single and unmarried, but it turned out otherway around, so you can not marry her. it is better for her as well to return back to her husband and kid. bu the way, is she away from her husband? do not marry her rather ask her to go back to her husband.