I'm 15 and my niece is 8. She lives with her mom but goes to her dads every other weekend. Today she told me that the last time she was there, her dad touched and licked her "butt." My niece doesn't know the difference between a vagina and a butt, so Im not sure exactly what shes talking about, but either way its disgusting because he's like 30 and she's EIGHT!!!!!! She told me to promise not to tell, but I need to because this is serious. What should I do?