Brother causing family breakdown

I am having some family issues and hope I can get some feedback and maybe some advice.
My half-brother K is 30 years old, and lives with my mom and step dad. Unfortunately I live about 6 hours from them, and I do not get to see my mom as often as I would like, sometimes only once every couple years. Time and finances are tight and I wish I could see her more.
The last time I went to visit mom seemed like she didn't want me to come, but I had already made the arrangements when she had second thoughts.
When I arrived I found out that my brother had been having problems with alcohol addiction, and had been swinging from rehab to hard drinking in a constant cycle. My mom was so stressed out, she was having problems thinking straight and seemed to be suffering from possible paranoia, obsessive thoughts and compulsive actions, as well as some memory, and judgment issues. I am kind of worried as some of her issues seemed to suggest some dementia as well. She has always been high strung, so I am attributing much of this to financial and emotional stress caused by my brother.
My brother, 30 years old, no job, living at home with mom and my step dad (his dad).' K' drinks, uses drugs, is constantly trying to get doctors to give him pain pills or anxiety medication. he makes my mom go buy him booze, makes her take him to the hospital to try and score drugs, because if she doesn't he is going to lose it and kill someone. He has tantrums on the livingroom floor, and my mom has to call the ambulance because of his high state of anxiety. sometimes when I call he is yelling and swearing and freaking out in the background. My mom is not allowed to talk about him, sometimes when I call he stands behind her the whole time to make sure she doesn't say anything about him! My mom just retired this year, and I can't help but feel what he is doing is elder abuse!
I was looking forward to being able to see my mom more now that she is retired and we no longer have to sync days off to visit. But in the current situation she does not want anyone to visit, mainly I think because my brother doesn't want anyone coming there. In the past there were holidays where my mom would not allow anyone to come because my brother didn't want them there.
My step dad wants to call the police and have K removed, but my mom won't allow it.
I have tried to educate my mom that her actions of enabling my brother are not helping.
I am at a loss. My mom's mental health is taking a nose dive, and the family is falling apart, and there is nothing I can do. I rack my brain and I just can't see any way of helping this situation

Sorry to hear of the situation. Unless your brother goes straight from a rehab to a sober living home with other recovering alcoholics and goes to AA daily, I doubt that he will get well.
We had a saying in AA that if you always do what you've always done, you'll always get what you've always got. Things likely won't change if he remains there and in fact, it is probably killing him.
Once his head clears, then they can treat other problems that he may have. It is hard to diagnose and treat a drunk as the alcohol will mask the other issues.
My mom and dad kept my brother at home. He virtually lived in his bedroom drinking. They both were convinced that it was schizophrenia and not alcoholism. Such was the family denial.
It finally took my brother beating up my parents a couple of times (they blamed themselves of course). My brother threatened a family member of the sheriff (that was the last straw), that the sheriff told him to leave the county and if he ever came back, he would be arrested.
This was the break that I was kinda hoping for, but I wish that it didn't have to be this way. Parents will always love their children, but this was keeping everyone in the family sick.
He has drifted in and out of VA hospitals and disappeared for days on end without anyone knowing where he was. He can't stop drinking or hold a job. My parents still send him money and support him, but we are now free of his presence and can move on.
Best of luck to you and your family.