Call the cops on that man. You can not allow this man to be in the same house with those girls, and it is clear that the mom is doing nothing about it. I was abused as a child and I can tell you that I wish someone would have helped me. I can assure you that that man would never admit to what he has done and never will. You need to have your niece checked out by a doctor but that does not mean there will be physical evidence at this stage of the game. But that would not stop me. I would do whatever I could to keep her away from this man. The reason I say call the cops is because once you take the child in to be checked they are going to go straight to the authorities and things will get really crazy. By you calling them ahead of time, you can make sure that they girls are not further tramatized by what will take place. I must commend you for stepping up for the girls. I wish I would have had someone in my life when I was going through it.
Oh my gosh, I am so sad for you and your family. You should definately take her to the doctor and they should be able to confirm what your niece is saying. I am sorry if I am over stepping my boundaries here but it sounds to me like the mother is in total denial. Of course the stepfather will not admit it. DO NOT let this go, this is so terrible I'm sick to my stomache. I can't even begin to imagine how you must feel. You should look at her bottom yourself as well. Have you noticed if it hurts when she goes to the bathroom?
don't give up on this, something has to be done. And again I am so sorry please keep me posted.
All my love.