How to tell my 19 year old daughter that I disapprove of her boy friend?

I disagree by far. Im 22 years old and im far from walking the straight and narrow path in life, but when my sister (and best friend)28 recently got involved with a 33 year old "old highschool friend of hers"(which she hasnt had any contact with for the past 117 years) I knew that even i, who had coverd up her doings many times and takin blame for many of her doings, even i had to 'back' my parents to get her oUt of there a.s.a.p! Cause he is badnews! When i met him i also thought that he had come right and he got his life together and and and. Cause thats what he told us. He made it sound as if hes the good guy in his previous relationship and i believed him! So at first when my parents start geting concerned and started getting "the feeling that sumthing isnt right" i would say to my parents:, "just give the guy a chance, we know about his bad past but lets just be fair to my sister cause she said that he has changed,so lets not judge him on rumours, for the sake of not pushing my sister away, just give him a chance" , cause i actualy thought, hes really a nice person when considered a 'friend', wich was true, but when it comes to his girlfriends.....its a way diffrent storie. And in a very short time, diffrent people were calling my parents and i to warn us about this guy. He has been in jail for assault, almost killing a guy. And his ex girlfriend (wich is the mother of his baby daughter) had to get an dissallowance thingy from the police so that he could leave her alone after 3 years of being abused by him. In the last argument with his ex he threw her of a flight of stairs after she walked in on him and 2 hookers and didnt want to partisipate in the act... He doesnt work and doesnt want to work either, he deals and uses drugs, is filthy, has a very bad reputation and has no moral bounderies wotsoever. Since he came into the picture my sister, who used to sleep 4 out of the 7 days of the week at our house(while she does have her own flat just 3 streets away from us), has visited us twice unpto today, and only visiting for a few minutes.then shes off again. She didnt call to just say halo anymore,when she did call it was onlyto ask if she can borrow my moms car, witch were quite often seen as neither of them has a car. And sms mesages were short and to the point. She tried to hide him from us at first because she knew of hes reputation and that my parents would most likely disapprove of him. But very quickly she got quite comfortable in making it clear that she sleeps at his house most nights,if not they sleep at her flat. And until now, my parents havnt even met the guy yet cause they just had excuses every time the occation arises when my parents invited them over to meet the guy.. After my parents got a call from my sisters boss saying that they are concerned 'cause she's gonna loose her job over this court case she has from being caught with drugs in my moms car a month ago.... (wich my parents didnt even know about!!) they started getting more concearnd and that same night they got a call from a shopowner in town (and family friend) telling my mother to come quick because this guy is pushing my sis around infront of his shop and she doesnt look right to him... so when we got to the shop they were already gone and since i know where he lives i showed my mother the way, just intime to find them(3guys and my sister) entering the boyfriends house(wich is also his parents house seen as he lives with his parents) My sis couldnt even walk straight. she didnt even recognized me or my mom. And after she refused to get in the car with us, my mother and i went to go get my dad whos bussiness is right arount the corner from where we were. I got in he car with my dad of again to go and try to save my sister from what shes about to unknowingly do to herself. After a big struggle but a failed attempt to get my sister in the car we left and called the police, who said there is nothing they or we could do seen that she is 28 years old. I for a fact know that that wasnt my sister i saw in the road, falling over her two feet, slapping my mother in the face and swearing and screaming brutally at my dear father who has never ever laid a hand on us, ever, and never will. I didnt know this person. Just 3 weeks ago my loving, friendly, inspiring sister didnt even drink, not even on her birthday! Mainly also because she always saw herself as on "standby" for her work just for incase they would call her to come in to help. Now she is screaming and swearing in the street and balling me and my parents brutally insulting names.... i new this wasnt right. When me and my dad came home we got to hear that this boyfriend called my mother and sweared at her over the phone saying realy discusting stuff to this woman he doesnt even know not to mention the fact that its his gfriends mother!! And the next day my sis didnt even know about this, and yet again she didnt believe us. He is known to drug girls so that they are totaly unaware of theire suroundings and then he and hes friends would have orgies with this intoxicated girl. Now tell me if i am wrong when i say that "Just the fact that there can be these disgusting rumours going around is just disturbing enough for anyone to "not want to get involved" with this guy" Right? And yet he's so very manipulating that he has my sister convinced that everything everyone is telling her is all lies. Even when the police busted down on my sisters flat after they've been watching him and following him there (she was at work, shes an ambulance perimedic) and found him and another guy and 2 prostitutes having an orgy in my sisters bed!and found drugs on him, She didnt even believed them! Cause boyfriend told her a diffrent storie. He has my sister so brainwashed that she now assured us that she is willing to give up her family and her work. she doesnt care aslong as shes with him. and this had all happend in a timeperriod of 3 weeks. Theyve been together 4 not even 2 months yet shes inlove, and love is blind.... so if shes 28 and can be blindly trapped in a situation like this, i deffinitely does not think that theres any age for that matter witch someone can be considerred 'grown up' or 'old enough' for her to make her own mistakes! That one mistake of believing this guy that first night and falling inlove can and properbly will cost my sister her life. Because now he has full control over her body and her doings and she doesnt even know it. And dont get me wrong, shes a clever girl who is a certified perimedic and were very much in controll of not only her life, but others lives asswell. And now her parents are the only ones willing to take the risk of losing their own lives and fight for her till the end. So parents, if something doesnt feel right, no matter what age your children is, dont hessitate, trust ure instincts, theres a reason for that feeling. and that strange feeling has neen caused by something. 80% of stories doesnt come from nowhere, it might not be that its pure truth, but it surely does come from somewhere. x0x0 a concearned sister