A lightbulb just went off in my head.
Autism is extremely hereditary and a low IQ is sometimes part of it. Did your son talk late? Was he developmentally delayed?
If so, I'd seriously consider that he could have autism. If so, he needs to have services to survive or he'll keep getting into trouble because he doesn't understand other people or, heck, the world. He never got supports growing up either...they had none when he was little.
You can help him get supports without living with him and without joining his drama. He has real, true problems, but there is help for them. He may not understand how to get those supports, but it's mostly just phone work, filling out forms, and visiting Aging and Disabilities. He is disabled, like your mother. He needs help, like your mother. She doesn't live with you, but she is cared for in a facility that serves her needs. Your son does not need a facility, but he does seem to need help with his life. There is nothing wrong with having a disability, but your son likely will never understand what other people do and he will not walk a good path without help because he can't.
There is no guarantee he'll accept help and that is his decision, but if I were you, I'd at least do that for him. And then I'd go live my own life once he is in the hands of others. If his IQ is truly 65 and his brother's three children ALL have autism, well, I'm guessing he is not just a criminal...I am guessing he is very in need of help.
I hope your grandsons are getting the interventions that were not available when your son was small.
Hugs and good luck!