Caught my older stepsister and my older brother having sex?

The other day I walked into my stepsisters(lucy) room to see if she had the stapler. She was lying naked on her double bed with my also naked brother max. I gasped and when they realised I was there and pulled away I saw maxs penis was slimey and lucys lowere half was covered in sticky white stuff. I ran out of the room. WHAT SHOULD I DO?