Mine isn't quite so dramatic as that but mil never liked me (or any dp's previous girlfriends or any of sil's bfs ever, so I know now it's nothing personal really).
Long story short though she was a bit of a nutter on ds's first Christmas. Totally took over and called me fit to burn when I suggested that some of the (six giant sacks of toys) should stay in her house until ds had grown into them as we didn't have anywhere to put them (think ride ons for an 11 week old). It ended with me sobbing and her calling me a bitch who didn't want her to have anything to do with ds (which was massively untrue, I had in fact repeatedly said I hope ds is as close to his nanas as I am to mine, ie, very close!
That was the main thing that kicked everything off but there was an unpleasant undercurrent of 'I've done this twice so I know better than you how to look after your baby' type of thing that seems to be so prevalent.
Dp basically stepped up, told her in no uncertain terms she had acted appallingly and wouldn't accept anyone treating the mother of his son in that manner.
She refused to admit she was wrong or apologise and we went low contact, only speaking to her if she called (and only dp).I tried to get him to talk to her in this time but he is just as stubborn and was having none of it
He said he had nothing to say to her until she wound her neck in.
Three months (and dps birthday ignored by his own mother in her stubbornness) she finally admitted to going over board and overstepping boundaries.
Amends were made and she realized that if she didn't behave herself she wouldn't get a look in, so while we'll never be close we have a reasonable relationship now (in fact the three of us are going to her house today)