What should i do about my friend?

Okay, last week, I was visiting my friend because we always study for a yearly test together on the last wk. I didn't call her, because I never do, and her family is like my family.
Well, when I knocked the door, no one opened it, so I just opened it and...

went in. No one was there.
I walked towards my friend's room, and it was shut. So, I opened it and saw something I should of never saw!
My friend, and her BROTHER who is 2 years older than her was having you know what on the bed.
They didnt see me, so I closed the door and I was SO shocked that I got out of there immedietly.
That night, I called my friend, and when she picked up, I asked her if I could come tomarrow. She said sure.
The next day, I went to her house as I said, and found them having it AGAIN on the couch! They tried to explain, but I just got out and went home.
2 days later, my mum told me to pik something up from my friend's mum.
I really didnt want to go, but I did anyway...
I got there and my friend's mum gave me the thing my mum wanted and she told me to have dinner with her family. I said ok, and said that my mum would b joining us too.
So, because I was bored waiting for the food, I went to my friend's room. AGAIN they were together, licking each others private parts. I wasnt that shocked, but they were.
They tried to explain again, but I didnt listen...

WHAT SHOULD I DO???

If I tell her mum, both of them are going to be DEAD literallly!

I seriously have NO idea what to do!
please help me!! THANK YOU!!!

oooh.. that's umm.. I cant imagine being in that situation. maybe you can ask her I mean if you guys are really that close.. and then if she says its what she and her brother wants, I guess you need to let them be (just advice that they need to have some sort of protection). I mean you cant force her to stop if she doesnt want to, right..

but that's really sooo weird. what if her parents walked into her room?!

Yeh I know!!!

Thank you sooo much!