From someone who is also living this drama, not just with 1 but with 3 adult SS's, 1 of which IS in prison. I can guarantee with 100% certainty you will never change them and you will never change your DH.
I suspect your DH absolutely knows and is aware of his childrens behaviour, but there will most likely be some past trauma surrounding him and or his kids, and your DH is terrified of upsetting them for fear of losing them!
I have litterally just come away from my very first therephy session where this subject is the reason for my stress and anxiety.
My DH's son's lie, manipulate, and abuse their dad, he knows it, i know it and they know it. I have tried for years to stop it, but all that happens is an arguement ensues, tantrams as thrown and ultimately the son's get away with their behaviour while i am left defated and hurt and blamed.
We have just agreed with a plan moving forwrds. I am removing myself from their lives. I love my DH with all my heart and it hurts to see the way they treat him, but, HE allows it! he's not forced into it, its a choice my DH makes. so moving forwards, after 13 years of this rubbish, i am stepping back. No more me sorting christmas presents, birthday presents, checking in, providing baby sitting, being a taxi and generally trying to be the glue. If DH wants to give/lend them money, they its has to be HIS money and not OURS. It's down to him now to communicate, arrange, organise. I'm not refusing to see or spend time with them, then if it suits me, i shall participate in whatever they are or want to do, i'm simply taking control over my own life.
You must make sure you know that this behaviour is not your fault, this is nothing you have done and it is not your mess to clean up.
Good luck