This happened to my wife and I about ten years ago while our first son was only three. At the time we were living in a two bedroom apartment. We had only been married for a few months when her sister (who i will call Claire) had some kind of conflict with her roommate (most likely the same problems we had). The two were ending their lease early and she needed a place to stay. Neither of us wanted her to move in but since she was family we worked out a price and she moved in a month later. Just to be clear, Claire living there wasnt what concerned us, it was her boyfriend who i will call Matt.
Matt was a bum. He was thirty years old, living with his mother, and dating my twenty-two year old sister in law. He had not been employed for nearly five years so his mother still paid all his bills. He mostly relied on Claire when he needed a ride to here or there, or she would just lend him her car while she was working one of her two full time jobs. So before claire even moved in we knew he would be a problem, and our predictions were dead on. For the first couple weeks he was only around when Claire was (an agreement we had made). While he was there he wouldnt socialize with any of us, he would spend the whole time playing video games in the basement where Claires room was. After a while he started finding ways and excuses for coming over while she was at work. He would show up unannounced at the front door on my days off with my son and invite himself in, usually bringing beer as a “gift” that he knew only he would drink. He would than go straight to the basement and start playing video games, often yelling things my son was thankfully too young to understand at other players. Several times we caught him stealing food and we are pretty sure he stole some of my wifes perscribed Xanax. He treated Claire with almost complete indifference even though he was largely living off her paychecks, he even had her bank account number. One night while i was on call for work i made the mistake of leaving my son in the living room while i changed into my work uniform. It was going to be a quick repair so i was happy for the excuse take my son to a close friend a few apartments down and to kick Matt out until Claire got off her shift in the morning. When i came back out a few minutes later Matt was asking my son if we kept a change jar anyhere. I dont think he knew id heard him.
These kinds of things went on for about a month before my wife and i decided to put or feet down. We basically told Claire that Matt was not to set foot in our apartment anymore. He was in the basement at the time and so she would have to tell him that night or we would. She was obviously angry about it but agreed to tell him herself. An hour later they were arguing loud enough to wake up our son. My wife was smart and decided to take him to my Moms house. By the time she got back twenty minutes later Claire was crying and Matt was still raging. I didnt hear what Claire said to him but we heard her say something and than a loud slapping sound followed by Claire crying. My wife an i were downstairs and on him a few seconds later. He took a swing at me and hit my jaw, but he was pretty weak so all he did he piss me off even more. The next thing i knew i had broken his nose. He was subdued but still raging. My wife called the police while dragged him outside. For the next ten minutes my wife was inside consoling her shocked and crying sister while i had matt pinned down face first in the snow, he was only in shorts and a T-shirt so i imagine the cold was a little painful, but he stopped struggling. The police arrived and after hearing the story they took him away. The Cop who talked to Claire was very compassionate and even recommended a good therapist who worked with victims of domestic abuse. They didnt even care that i had all but shattered his nose Claire had to change her account numbers and spent some time in therapy but had much better boyfriends thereafter. Im not entirely sure where Matt is these days but im guessing he still lives with his mom or possibly in prison.