"I Saw My Son And Daughter Having Sex"

Last night I heard noises in my son’s room and I opened the door and my son was having sex with my daughter! He is 15 and she is 21 and they didn't use protection.

I never expected that, never thought they would do such a thing. I just don't know the right words to use, I am still in shock. For how long have they been doing this?

This is both a sin and a crime, and if anyone gets to know about it they could arrest my daughter.

Please Help. I need a wise advice!