What should I do if inappropriate things happened between 6 and 12 year old?

[Anon781]

My 7 year old niece of whom I am the legal guardian of told me that when she was 6 years old  my 13 year old stepson ( at the time of the allegations he was 12 ) tried pulling up her underwear to expose her intimate area while she slept. On another occasion she said he grabbed her hand and made her touch his penis over his clothing. On a third occasion she said he exposed his penis at her. I don't know what to do. Why would she wait more than a year to tell me this. She never showed any signs of trauma of any kind that would make me suspect something of this magnitude. Her and my stepson get along fine. She's always asking him for help with things. She's never shown fear of him or showed she was uncomfortable around him. So why now say this after more than a year. On one hand my niece tells me she's telling the truth and on the other my stepson says he never did anything. He says he's innocent. What makes me a little confused is that when my niece talked to me she first just told me about the 1st incident. Later on in the day she came back and told me oh wait I remember more and that's when she told me about the other 2 incidents. She keeps changing her story, but says she's telling the truth. This is tearing my family apart. After hearing my niece's allegations against her son my girlfriend packed her things and her and my stepson left. Please help any suggestions would be really helpful. What should I do?