Advice on a toxic adult child

I’m posting this for my mother since she doesn’t have reddit or any social media and is losing her mind.Backstory- my sister is 22 and a complete lazy loser. She’s never had life hard but yet complains she has. She moved with my parents when she was 18 and got her GED in the town we live now. She smoked a lot of pot and played video games, nothing too serious. Was never interested in getting a job or driving. Well fast forward 2 years she is 20. Still doing the same things. Well she meets a almost 50 year old man who is a junkie and befriends him. They are now in a relationship and she leaves home to be with him. Gets addicted to meth. She continues to ask my parents to support her and asks for money food etc. lies constantly about being with him so she can move back in because he got arrested. While he is in jail she moved back in and was just a lazy slob. Didn’t contribute anything and made the living room into her own room. Right when he got out and came home she was gone and with him. My dad got so angry he got arrested for starting an altercation with this old meth head. Well ofcourse my dad gets out that night and my sister is back with this loser. Now she and him texted my parents saying they need so much money or else they will up the charges against him. Basically blackmail.Well the charges got dropped eventually and my parents cut ties with her while she was away with the guy. Well the guy had to go to rehab as part of his probation agreement. She goes with him. They continue to do drugs. She gets kicked out and he stayed. She had no where else to go ofcourse so at 2 am she knocks on my parents door. They take her in. This was about 6 months ago. At first she was seeming to want to do better. She went to work with them (they own their own contracting business) , she did chores, etc. It got worse. She was stealing their beer, lazing around all day, drunk constantly. Has no intention of getting her drivers license so she could get an actual job. And is very disrespectful to my parents.My issue is, they still will allow her a few beers here and there. Rather than cutting her off and dealing with the monster she is when she’s sober.So yesterday is when I’ve had enough of her disrespect towards my parents.My parents were gone and she’s home alone. She drank and was drunk by 3 pm. Almost all of their beer.(They are beer drinkers)I’m so beyond annoyed by her and annoyed by her actions. I’ve told them so many times to cut her off from beer and she can’t smoke pot in their home anymore.My mom is at a loss as well.We live in a rural area so kicking her out is not really an option. Rehab?Literally posting this for any inkling of advice from someone from the outside looking in. I can only type so much but she has done so much damage. via /r/Parenting https://ift.tt/2uXBxV3