9 yr old daughter says step brother is in her bed but doesn't know if she is dreaming or real?

My daughter is 9 years old and my stepson is 14, my daughter told me yesterday that she dreams that her stepbrother is in her bed, but she thinks it is really happening. I confronted my stepson who denies it. However 3 years ago I did wake up and find him in her bed, My husband and I explained the innapropriate behavior and treated as an isolated incident Could this be happening again? what to do?