My sister and her boyfriend were having sex on my couch?!!?!?! HELP!!!!?

i was sitting on the stairs to listen for anything suspicious, because my Dad trusts me to watch them when he goes to work and other places, so i pretended like i was going upstairs, because i heard a noise like someone using lotion, so i sat down on the stairs and after about 5 minutes i started hearing heavy breathing, and the couch rocking. then later they started talking, i wont go into details because im still traumatized by what i heard. But I want to tell my Dad but i think he'll be mad at me for not watching them close enough, or her boyfriend will beat me up because he's a big guy. Someone please help me im scared, and nervous. Please!!!!!!!!!!!!!!!