Hi, I'm looking for some constructive advice about how to deal with this as it's been going on far too long and I don't know where to turn. Sorry in advance for the long post.
I have been with my partner 17 years and we have 3 wonderful children together.
I never warmed to DP's mother. I always found her intimidating and false but couldn't put my finger on the reason why. It turns out she is very self centered, manipulative, and ticks many boxes for Narcissistic Personality Disorder (I have done lots of reading and don't use this term lightly).
Not long after meeting her, the snide and bitchy comments and sniggers towards me began, normally when no one else was around to hear. I always assumed I'd done something to deserve it and tried harder and harder to get her to like me. Over the 17 years I have known her she has become worse and worse. She has deliberately humiliated us in front of her friends and family, organised an expensive family holiday and ruined it by setting up arguments and sulking. She implied to my face that i was incompetent, implied to DP the I am a freeloader (I have had a job for the whole time we have been together). She has put extreme pressure on us to get married- not because of her moral values but because she wants to be the centre of attention and brag to her friends. She manipulated DP into agreeing to lend our car to their friends from overseas whilst we were away (I'd never met these friends) because she knew I'd say no. She spent my pregnancy telling me I was going to have an enormous baby, then tried to name our child.
Since our children came along, she undermines us in front of them, gleefully ignores our values, and is obsessed with getting them away from us to spend time alone with them. When we try to set boundaries she either goes into victim mode and cries/sulks, or sees it as a challenge. When she doesn't get her way she creates horrible atmospheres, even in front of the kids. She has been rude to my 4 year old for talking about something fun he did with my family- I think she was jealous. Things escalated last year when I tried to set a small boundary with her in the politest way I could. Afterwards in private she had a melt down, cried, called me aggressive and tried to get me to apologise. She also told me I can't involve DP in the disagreement because it's none of his business. Her family members have stopped communicating with me on social media so I think she has been bad mouthing me to them.
DP knows her behaviour is awful but has been conditioned by this woman not to stand up to her for fear of retribution in the form of a guilt trip/tears/hissy fit. We have reduced our contact with her because of her behaviour, and now she is trying to force he way further into our lives. We now avoid leaving the children with her unsupervised but this isn't a long term solution as it week be harder to do as the kids get older.
I don't want my children poisoned by the bitter old bag, or used as pawns to get at us. I worry that she will try to turn our children against us, I know she is capable. Help! 
AIBU?
To want to exclude DP's mother from my children's lives?
Weareallmadeofstars · 22/01/2016 21:10
Simple- tell the poisoned bitch to fuck off!
You've tryed for 17 years ffs, you've done your fair share 
Firstly thank you all for the great responses, there is some fantastic advice here 
Ifrit
From experience, people like this go into victim mode when you try to call them on their shit and they twist it so that you're the bully. You can't argue with that level of emotional fuck-up.
Yes that is exactly what has been happening. I have tried to call her on her behaviour and I always come off looking like the nutter 
She is scarily clever but I suppose she has had 67 years of practice at being this unpleasant
Toomuchtooold
That's good advice, yes our primary aim should be to protect our little ones from her poison.
She is anyways one step ahead of us and only she knows the rules to her games. I'll think I have got out of one game only to find myself in the middle of the next. Is mind boggling.
It has taken DH a very long time (and lots of long conversations, tears and arguments) to accept that his mother is this messed up. He has been making excuses for her all his life and will do anything to avoid confrontation. Don't forget he grew up thinking she is normal He forgives and forgets and he preferred it when i did the same. Now he has accepted that she has problems and agrees that we shouldn't have to put up with her behaviour, but hasn't a clue how to address the situation. We talk about solutions and strategies, and he agrees that next time she's horrible he will stand up for me but it never happens. Though more recently he has been venting to me when she upsets him (instead of pretending it never happened), which is progress.
I think it will be a while before he agrees that NC is a good idea. I am worried about pushing him away if I give him an ultimatum. We can't let this woman come between us any more than she already has. We have to be strong together and that's what I'm working on at the moment. It's a long process.
We have started keeping school events and family holidays secret because she keeps trying to invite herself along. Consequently this years family holiday is a secret from the kids so they don't let slip to Grandma. Not healthy and sooner or later we will be caught in a lie.
We have never let her have the kids overnight or pick them up from school, which she is upset about, especially a her friends get to babysit their grandkids all the time. We have stopped leaving them with her altogether, though this doesn't stop her trying to take the kids to the park 'so we can have a break from them' 
when we do see her. I'm always close enough to hear what she is saying to the kids so I can deal with it if she's out of order.
If we have to see her I try to meet up at the park, go for walks etc which makes things easier in a way but she spends the time trying to go off with the kids e.g. take one of them to the loo or deliberately lag behind with them if we are walking.
I'm just exhausted with the whole thing and spend too much time thinking and stressing when I should be enjoying family life.
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