My little sister statutory raped herself

IDK if anyone has any advice at all but a friend of mine could get in a lot of trouble. My 15 year old sister was over because she got in trouble with my mom and my mom needed a break from her. One if my friend's who's 27 asked if he could come over because he was feeling suicidal. He came over and we talked and Vaped. I told him if he felt like he was gonna kill himself he could sleep on the couch. He took some medication and was drinking some sky vodka I had. I asked if he's supposed to be drinking with that medication and he said he's only gonna do it this once. So I went to bed.

Well my cousin messaged me said that my sister has been going around telling people she slept with my friend. I went to my mom's house to talk to my sister. I asked was it true and she smiled and said yes. She told me he was asleep on the couch and he was hard so she tried giving him a hand job to wake him up but he didn't wake up and then she... Ya know .. I asked if he asleep the whole time and she said yes. I told her that is rape. She fucking goes "Well are you gonna tell on me?" Idek how this situation would go in court. IDK what I'm supposed to do. Two peoples lives could possibly be ruined. My friend's or my sister's and either one would be awful