for those who think it's safe

I have a confession.

My friend Troy has a sister who has downs syndrome. She’s 4 years younger than me, although you’d never know by looking at her. I’ve always been real nice when she picks up the phone or I go over there to hang out. He’s already told me that she thinks I’m nice, something she doesn’t think about most boys that come over there.
Cut to Monday night. We all have Tuesday off for Thanksgiving, so Troy and I are over at his house (he still lives with his mom) playing some Xbox. His sister is in the room, playing with dolls and occasionally watching the screen as we played. Troy’s mom is out running some errands; I think she was at her mother’s house. The phone rings at about 8, and it’s Troy’s mom, Victoria. She tells Jessica (Troy’s sister) that her car has broken down and needs Troy to take a look at it. Troy, being a car mechanic leaves to go check out the car, and asked me to look after Jessica while he was gone, since she gets really upset by cars. I agree, and he leaves.
I didn’t really know what to do to entertain her, so I just sat around and kind of fucked around, and would occasionally talk to her. For the most part, she sat there and played with her dolls, talking to them. I was sitting at the computers working on an essay for an Archaeology class when she asked me the following:
“What are doing Mike?” “Oh, I’m typing a paper for a class.” “Heeee you need paper for class!” “Yep, it’s pretty important.”
She trailed off for a minute, and I resumed typing. About 5 minutes later, she spoke again.
“Mike, I know sex” - I was taken back. What do I say to her? Do I lie, do I acknowledge that she “knows” sex, what? “Um, I’m not sure if we should talk about that, Jessica.” “I try sex with me and it good” “Jessica, we really shouldn’t talk about this…” “Will you try with me?”
I didn’t know what to do. I was sitting there, with my mouth half opened and I’m sure there was a puddle of drool forming on the carpet pretty fast. She asked again. “Will you try sex with me? Nobody like me like you, you so nice and pretty”
Fuck. Fucksticks. Fucking fuckersticks from planet fuck. I’m getting hit on by a retarded girl. What the fuck do I do?
Yeah, I fucked her. It was her first time, and she bled. It was not pleasant, and she wailed near the end, I guess an orgasm. When we were done I went to the bathroom, cleaned up, and cried. I fixed up myself and made sure she was alright. Then I went back to my essay while she did god knows what. I didn’t care, I still don’t. A wave of apathy crashed upon me.
Troy showed up about 10 minutes later, and I quickly excused myself to go home. I went home and cried myself to sleep on my pillow, accompanied by the horrible imagery which will haunt me to my grave. I woke up yesterday to the phone ringing, at about 7:30. Most people know to leave me alone at 7:30 when I’m not working, but I rolled over to see the caller ID. I looked down at it, saw it was Troy’s house, and answered.
“Hey Mike, I need to, um well…ask you about…uh…something.” Me, suddenly awake, “Uh yeah, what’s up?” “Well, last night after I brought my mom home, Jessica was acting strangely.” “Oh, how do you mean?” “There was blood dripping down her leg.” “What the fuck?” “Yeah, that’s what we thought, too. We are trying to figure out what happened. Do you know anything?” “No man, I don’t. That’s pretty fucked up. Say, I got to go because my breakfast is burning.” - lie “Alright Mike, I’ll talk to you later.”
What should I do!?? Confess to him and his mom about Jessica, keep my mouth shut? Threaten her with violence if she talks?