How do I get my friend to knock off this “prank” ?

I live in a 5 bedroom house, and I have a friend staying with me for a few days. My sister also lives with me. She’s 18 years old, and she’s a little bit of a weirdo. My house has an intercom system that attached to an old alarm system. My sister likes to play around with the intercom at night, and pretend she’s a serial killer.<br><br>She’ll just ramble on random stuff like “I’m in the walls, you’ll never catch me” Or “I’m standing right behind you, and I can see everything you’re doing”<br><br>My friend recently found out that the intercom system works, and he thinks it’s the perfect way to scare my sister. So every night, he’ll go on the intercom and pretend to be a creepy guy in her room.<br><br>The first time he did it my sister flipped out. She had just gone to bed, and my friend goes on the intercom with this really deep, creepy voice and says “I’m in your room, I can see you”<br><br>My sister came into my room in tears after that, absolutely hysterical. She told me that she was positive there was someone in her room, and that she saw one of her drawers slightly open.<br><br>Of course I didn’t tell her my friend had done it, but I made sure he knocked it off. It wasn’t funny, and it was mean. Especially since my sister has a fear of home invasions, she’s always worried about people getting in the house and hurting her.<br><br>The next night rolls around, and my sister is fast asleep. My friend grabs my arm and tells me to listen, and then he goes on the intercom. This time he changes it up a bit, and says “I’m in your closet. I’ve been in here for weeks, and I know everything about you” <br><br>My sister comes running into my room again, absolutely hysterical. This time she told me she could smell something weird, like someone had sprayed cologne in her room.<br><br>I told my friend to knock it off, but the next night he does it again. This time he says “You’re my girlfriend, I’ve been watching you for years and I know everything about you. I know you have a crush on your best friend, and I know you think about killing yourself sometimes”<br><br>I’m not going to repeat everything word for word, because my friend can get pretty graphic. Let’s just say he really went off the rails with this one. He also told her that he’s been living in her walls for months, and that he’s been sneaking into her room at night to watch her sleep. He said he’s been sniffing her underwear, and that he’s touched her a couple of times while she was sleeping.<br><br>My sister lost it when she heard that. She was so hysterical that she couldn’t breathe, and she threw up on the floor. I scolded my friend for doing it again, and told him to just let it go. This wasn’t funny anymore, and he was really messing with my sister’s head.<br><br>The past couple of nights, my sister has been waking me up in the middle of the night and begging me to check her room for her. She’s terrified that there’s someone hiding in her closet or under her bed. She’s even asked me to sleep in her room with her, which is not something she has done since we were kids.<br><br>I’ve been reminding her that she knows it’s just a prank, and there’s no one in her room. But she’s just saying that she really feels like someone is in there. She’s smelled the cologne a couple more times, and she could swear that she saw someone moving in her room out of the corner of her eye.<br><br>Last night was the worst night so far. My friend didn’t even go on the intercom, but my sister woke me up an hour after I went to bed. I was groggy and half asleep, but when I heard her screaming I immediately jumped out of bed.<br><br>She was standing in my doorway, and she looked like she had just seen a ghost. Her eyes were wide, and she was shaking like crazy. She kept telling me that there was someone in her room, and that she found a long piece of hair in her bed. She was telling me that whoever was hiding in her room must have fallen asleep in her bed while she was gone, because the hair wasn’t hers.<br><br>I searched her room for her, but I couldn’t find anything. No sign of her alleged hair, and no sign of an intruder. I even checked the vents, and I couldn’t see anyone hiding in the ducts.<br><br>My sister was just inconsolable. She was crying so hard that she was vomiting, and she kept telling me that she knew someone was in her room. She said that whoever it was must have hid in the walls again, because she saw him. She said he was tall, skinny, and had a young looking face. She said that his eyes were black as coal, and his teeth were small and pointed.<br><br>I don’t know what to do. I’ve begged my friend to stop, but he just laughs it off and tells me it’s funny. I’ve told him that he’s really messing with my sister’s head, but he just thinks it’s hilarious. I don’t know how much more of this my sister can take. She’s really starting to lose it over this stupid prank. <br><br>How should I get my friend to knock it off?