I am so sorry @AnightwiththeTiger that you are having to deal with this situation.
I would tell my child that we have something important to discuss.
This is not an off the cuff, spur of the moment conversation, you need to prepare and have appropriate arguments. Write a sort of schedule to stick to so your emotions do not trip you. Deliver a supportive, simple and clear message.
Use specific examples to discuss how your mother's behaviour affects the family. make him see how his siblings, his cousins, everyone, is impacted. Review with him this 'special relationship' she has with him, understandably, from his point of view being the special one night not be so bad, but children have an innate instinct about what is right and what is wrong and they understand that putting boundaries around a person's bad behaviour makes the world a safer and fairer place for everyone.
Be open to questions and also about the fact that this is a difficult and unusual situation.
Encourage him put his own words and thoughts on it all.
Explain to him what the consequences of their behaviour are for this person and how your family is going to deal with this.
Make clear to him what the procedures are from now on with phone calls, social media, visits, holidays, family reunions, the lot. Absolutely stick to this.
He needs to know that it is your responsibility and that you are making this decision for him. Watch out for any sense of guilt he could have or that could be encouraged by you-know-who.
Yes, this is a bereavement. You are burying the hope that this person can ever be a normal, caring mother and grandmother.
At 12 you only have a narrow window of opportunity.
Act quickly, or watch your mother have an absolute ball 'supporting' your DS in his teenage years,. This is not a drill.
No one would ever wants to have to think the way we have to. There is no way you can make this situation palatable or easy, or even have the perfect conversation with your DS about it.
Take practical steps to make 100% sure that your mother cannot contact him in any way without going through you. If only it were, as @Thosefuckers thinks, a simple matter of laying the law! Be vigilant and take into account that she is devious, determined and has probably little else to do. If and when she does contact him, stamp down immediately on anything inappropriate and explain to your DC why it is inappropriate.
Unlike you, I did not understand my mother's personality. I was misguided enough, as@Thistlelass is, to think that I should not deprive my DC of, or 'spoil', her relationship with my mother.
I completely failed to recognise we were in a war. My DC is now paying the price, living every single day with the results of my mother's abuse. Do not make the mistake I have made.
@AnightwiththeTiger there is no reason WHY she is incapable of being happy for you, or have a reasonable relationship with you and your DCs, she just isn't capable of it. It takes a lifetime to accept to see how grotesque these personalities are. I hope you make the most of your painfully acquired knowledge.
Good luck. 💐