honestly, her moving away will be the best thing ever for you!!
You and your DD are now safe. She was an AWFUL mother to you and your sister and is STILL awful, but bitching about your little girl too - out of fucking order!
My own DM did and said some very weird things to my son before she moved away - but as she didn't actually tell me anything about her move, deliberately excluding me, I left her to it. I did ask her why she had excluded me from her move, and was told that she and I 'were never that close'
. that STILL astounds me!! We were in touch every day, she'd pop in to mine if she were in the area... it was completely out of the blue. Man it hurt!
I know the anticipation of this is going to upset you, but it'll be OK on the other side.
You have had some amazing advice here, you are NOT responsible for her happiness, and the idea that she is selling her house and going through the enormous effort and expense to do so because of little old you is ludicrous! Your entire life, your DM (and indeed your DF) have shown consistently that YOU don't matter a jot! So why on earth would she be doing this for any other reason than for her own self now?
She is a nasty, abusive woman, a piss poor mother and fucking awful grandmother. She blights the lives of all she knows. She is managing to take a decision to move house and use it as a weapon to hurt you.
ONLY IF YOU LET HER.
Grey rock the shit out of her - 'Ok mum, good luck with the move, hope it all goes well' and leave her to it.
That's what I did with my M too btw, drove her nuts, she recruited my Sis to guilt trip me - all ignored. I had a therapist at the time. It was enormously important and helpful.
If you can't access a therapist, please get yourself to the Stately Homes thread on Relationships - we'll all know what you are feeling and why.