I have no idea what is going on, so I am asking you for your thoughts. Sometimes it is easier when you are on the outside of a situation, right?
Not sure where to start, and don't want this to be too long, so bear with me?
DS and I always had a good relationship, despite my mother undermining my parenting constantly.
I brought him up as a single parent. When he was in his teens, I met someone, and straightaway, my mother vowed to do all she could to split my partner and me up, she criticised everything my partner did, and everything he didn't do.
Despite this, my partner and I continued seeing each other.
After a few weeks, my mother got together with my sister and phoned DS's school without telling me, and made some serious completely unfounded allegations about my partner, which almost had DS removed from my care.
From their perspective, they said they honestly thought they were protecting DS and could see through a crystal ball that DP was no good for me. Mother told me that she was in full support of my sister reporting their concerns as if they were fact when indeed they weren't because she truly believed DS would be sent to live with her, something she has always appeared to want but has never admitted, so maybe not, just one of my many confusions as to whether she is actually telling the truth or not.
Anyhow, luckily the school made an evaluation and against all protocol made the decision not to contact CP, and sent DS home after school as usual.
Since then, things were very strained between myself, mother and sister.
From that point onwards, DS discovered that he could play me and my mother off against each other. If I set a punishment, DS would wait until he was alone in the house, ring my mother and mother would drive over while I was out, pick DS up and take him back to her house for a few days.
I repeatedly asked mother not to do this, until I was blue in the face, but she said she couldn't bear to see DS upset, and was giving me a break. 
I told her I didn't need a bloody break, I needed her to stay out of it, but she said she couldn't sit by and do nothing when DS rang her needing her help FFS.
I had to work and couldn't physically force DS to accompany me everywhere when I wasn't at work, so this pattern continued, despite my protests.
I was getting more and more frustrated and annoyed with the complete lack of support from my mother, who incidentally thought she was being a great GM by supporting her GS, and it did get to the point where although I tried not to, I was walking on eggshells to some extent around DS because of course the slightest upset, and off he'd go again.
Things came to a head when DS was 16, (old enough to leave home).
I had had enough of this complete lack of respect from DS AND my mother for me, and I honestly believed DS would not enjoy living with mother full time, I had lived with her myself.
So I made it clear to DS, since mother just wasn't taking any notice of anything I asked wrt this situation, that the next time he asked mother to pick him up so he could disappear for a few days, he could stay at hers, and it was not on for him to keep ping ponging back and forth as and when it suited him to avoid doing anything he didn't like.
I told him he could visit as much as he liked, but I was not prepared to be manipulated like this any longer and if he disappeared without asking again, I would take it that he had made the choice to live with my mother full time.
He went. 