So GFG17 is mad at his sister (age 20) because she had a conversation with his girlfriend while they were broken up a couple of weeks ago. Girlfriend told my daughter that difficult child had hit her on the back once when they were fighting. Daughter told difficult child to cut it out and work on his anger. So now difficult child is pulling out all the stops to get "revenge" on his sister for talking to the girlfriend and "getting in his business." He and girlfriend are back together, by the way. They've been dating off and on for two years. She's a lovely but naive girl who always takes him back although he is very jealous, controlling, etc. (when he's not being nice and charming). So for the last couple of days he's been threatening to do various things to his sister. He said he's going to put all her things (clothes, etc.) on the back porch or in the alley. They're currently in boxes in his closet, as she lives in a college dorm. I put everything in there because he doesn't even use the closet, but now he wants to put all her things outside. Then he said he was going to contact his sister's former boyfriend and tell him her new phone number and which guys she's been dating. She has been to court twice to get a restraining order against this jerk, who has been threatening and harassing her for a long time. She's had her phone number changed twice and has finally succeeded in no contact with him for over a month. This guy also calls, texts, and harasses anybody that he finds out she's been dating. Fortunately, the ex-boyfriend lives in another state, but that still won't stop him from possibly doing the stalking thing again if he gets her new phone number. I told difficult child to just stop it, don't be a jerk, and why can't he just let it go if he was upset with his sister. He said, "That' s not how it works, Mom. She's gonna pay for being a dumb ****. I'm not going to tell you everything I have planned because I can't afford for you to call the police." He still wants to join the military so wants to not jeopardize those plans. My daughter is relatively stable but has some issues herself, has been in therapy for over a year dealing with the toxic ex-boyfriend, and doesn't hold her anger very well with difficult child. She's the one with the rather superior attitude toward difficult child, so they've already got some negative history. They're my two youngest, so although can be very close and supportive to each other at times, they always have an underlying tension. Opinions, please? Should I just ignore difficult child and let him do whatever and let daughter deal with it? I don't know if he's serious or just popping off. But he's capable of doing some mean and stupid things. Should I warn my daughter that difficult child has something up his sleeve? Should I wait and see if he actually follows through with anything and just react as I see fit? I told him this morning that if he does anything hurtful or damages any property, I'll call the police. I considered even renting a storage unit for my daughter's things so he can't use that as a threat. Not really in the budget, but might be worth it.