Totally agree MammaTJ.
DS lived with me until he was 16, and I got so fed up of him disappearing to DM's when it suited, that I eventually told him if he did his disappearing act one more time without telling me he was going before he went, then he could stay there.
He waited until I had nipped to a neighbours for a cup of tea one evening to disappear again, but I came back sooner than he'd expected and found him packing a few things. The next thing I knew, my mother was on my doorstep, and when I asked her what was going on, she said it was nothing to do with her. I told her it bloody well was because she was picking him up against my wishes, and she just repeated it wasn't anything to do with her.
I tried talking to DS, asking him to think about what he was doing, that he could not just come back in a few days as before. We had already been over that, but I told him again, but my mother just ushered him out of the door and sort of propelled him to her car, away from what I was trying to say. 
They got in the car and drove away. A few days later, my mother parked 3 streets away while DS came to collect his belongings.
I also tried to pick him up from college and asked my mother not to go to the college because I would be picking him up that day. When I arrived, there was mothers car parked outside the college. DS again chose to go with softy nan.
It was a long time before I saw or spoke to DS again. He blocked me on his phone and I cut DM off.
Eventually, DS began speaking to me again, because I persevered in trying to contact him, but our relationship had changed. I felt betrayed by DS, and I think he felt betrayed by me tbh because I was pg and he had been told by DM and Dsis that he was not a part of my new family anymore. 
I tried to tell DS this was not the case, but the seed had been sown.
He was at DM's for 2 years and in this time, he began smoking, drinking and was falling behind at college. He was expected to do nothing.
He wanted to return home 2 years later, and it was then I discovered he had a drug habit (which he stopped when he returned because he was too far from his suppliers. I live out in the sticks.)
He asked me to attend a parents meeting at college where I found out he had gone into college every day wasted for 18 months and was unlikely to pass his course.
I agreed to let him come back after 2 years because he was a mess and I honestly believed he had learnt that living with DM was not what was best for him. silly naïve me.
He stayed at mine for a year, knuckled down, passed his college course, started driving lessons, got a good job, stopped taking drugs and only went out at weekends drinking, which was a vast improvement. Also he was 18 by now, so legally he could drink.
Mother didn't take it very well at all, and cried constantly, said she would never let another man in her life, she spoke of DS leaving hers as if he was her DH, and the whole situation was reminiscent of a DH leaving his wife for his mistress and then returning to his wife. Mother was completely distraught, inconsolable, said she had never been so hurt in all her life. She couldn't sleep or eat, and she went over and over every little detail of her life when DS lived with her all of the time.
Mother told me she had done everything for DS, he hadn't had to lift a finger, she had allowed his friends to stay, had bought him alcohol from the age of 16 had given him money, had felt threatened by his friends, been frightened of DS, he had been rude to her, called her names etc and she had tolerated all of this yet he had still left her. She said she would never let DS in her house again, so hurt was she. She denies she said she didn't want him back in her house now, but apparently she said she would never have him overnight again.
A year went by, DS straightened himself out, contributed to household chores, got his finances sorted out and distanced himself from my mother, but she never stopped obsessing about him.
A year later, he moved into a house share, where he stayed out of trouble. That lasted a year, then he went back to my mothers.
Well, she wont see him out on the streets you see.
He has been back at mothers for about 6 months and already his friends are dossing there every weekend, in fact some weekends, DS doesn't return to hers, but his friends turn up for a bed and she just opens the bloody door to them.
It's all pretty much as it was before, DM does everything for DS, and even asks DS if it's ok for DD to go and visit. 
She waits on him hand and foot. Not only that, but she wont hear of DS doing anything wrong.
DS has now lost his driving licence for being in control of a vehicle while under the influence of alcohol, mother has paid his bloody fine and drives him wherever he wants to go, even if that means picking him and friends up at 4.30am after he's been drinking.
He has also thrown a plant pot at a neighbours car windscreen as a joke 
and mother agreed to not tell me, and to protect DS by lying that he was at hers that night if the police came, and the latest is that DS is on bail for an arson attack on someones house, and for theft of a garden ornament, yet DM says she doesn't believe DS was involved, the police have just picked on him because he was involved in breaking and entering an empty pub last year.
The police have been searching DM's house for the garden ornament, and where was DS when these searches were going on?
Getting drunk with his friends, and of course DM wont tell him to come back because it will spoil his weekend surely??
I want to shake her and tell her to stop enabling him, let him face the bloody consequences of his behaviour instead of paying his fines and writing stupid letters to the court to get him leniency, but she just keeps going on this crusade to make life as easy as possible for DS regardless of what he does.
At this rate, he'll be in prison by the time he's 25. I have told mother this. Mother doesn't tell me half of what goes on. I don't think she likes my suggestions as to how to deal with it, which are along the lines of making him face the consequences for his behaviour!!!
I know I am better off out of it, but a part of me says this is my DS and I don't want his life ruined because he felt he was untouchable by getting away with so much. Another part of me says 'let them get on with it, just keep my distance'.