MommyBird I could have written your post as well.
For what it's worth, before I married DH I had three previous relationships. One lasting two years, the next lasting three, the last one four (DH and I have been married now for over thirteen years, just in case anyone is expecting a five next 
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I got on well with all three of my 'almost MIL's. The first one has now sadly died but we always used to exchange cards at Christmas etc after my relationship with her son ended. The second one, I still see fairly frequently, she gave a very generous gift to my son when he was born, and we still get along very well. The third, I don't see anymore but I miss her and whenever I see my ex I ask after her and he tells me she still asks after me.
And when I met DH, I was really pleased that he came from a large and (seemingly) close family. I was more than prepared to be part of it, I was looking forward to family get togethers and visits, lots of gatherings, I would phone MIL as often as I would phone my own mother, just to have a chat etc, even having them stay over with us often.
In short, I was happy to do everything MIL has always claimed she wanted in her family, have a big, family focused social life, visit them often and have them visit us often as well.
And on the surface, yes, we are very alike. Similar interests and hobbies, that sort of thing. We should have been good friends.
Instead MIL, supported by FIL, has strangled every relationship she has with people, including her relationship with me.
There were so many things that they did but MommyBird has said a good deal of it.
She's spread lies about me (and her other DIL) and played one part of the family off against the other. DH and his siblings have fallen out frequently over things she's told them about us (usually me) or us about them (usually her other DIL). One BIL once sent DH a stream of abusive texts calling me a 'fucking mental bitch' because she had made up lies about me.
She's told total strangers my medical information. These are things we couldn't help but have her know, not things I could have kept private and chosen not to tell her. We had a stillborn baby and MIL chose to ask me, three days after his birth, if it still hurts to give birth if your baby is already dead.
And then in my next pregnancy I was badly injured in a car accident and my baby was born prematurely and died as a result. MIL had a friend who was a midwife and told her we had lost the baby but not why, then decided that the most appropriate thing to say to me the first time she saw me after our daughter died was "we know what's wrong with you" because he midwife friend had diagnosed me with pre-eclampsia. They then threw away our photo's of our daughter and asked if she was born with all her face or with bits missing.
Told us our children were not "proper grandchildren' because they had died. Threw a tantrum at our son's funeral because DH was "being miserable" and they were offended that he sounded unhappy when they spoke to him on the phone the night before.
And yes, tantrums if not getting her own way. Tears, weeping, nasty comments, lies spread around the family, telephone calls fifteen or twenty times in an hour, messages that are just weeping, FIL ringing up to shout, both BILs ringing to shout at DH, SIL (DH's sister rather that the other DIL) ringing up to ask us to sort it out, more tears, lots of "you must think I'm a terrible person but I don't know what I've done. Why do you hate me, how can you do this to me, I've tried and tried with you, what can I do now, I don't know what's wrong, you all hate me." All because MIL wasn't getting her own way about something.
And then in the background, spiteful messages, half-truths, outright lies, guilt trips. Harassment like you wouldn't believe. Disowning DH one minute then claiming it never happened the next. Following me in the street, almost running me over with DS in his pram beside me. Sitting outside the house in their car day and night, never leaving us alone for a minute, trying to make DH leave me for a woman who wouldn't give birth to dead children. Trying to split us up so they could try and get access to DS through DH without me in the way. A barrage of texts and messages, half of them claiming to want to make things up and the other blaming me for everything and calling me names. Making threats to track us down and force me to do as I was told.
DH grew up all his life being told that if he ever, ever said no to her about anything she would have a mental breakdown and it would be his fault.
DS and I are NC now, DH sees them infrequently and with various results. They are toxic to the extreme but if you asked them their side of the story I suspect they would pass a Jeremy Kyle lie detector with flying colours, so convinced are they that they have never and would never do anything like the things I have described.
They made me ill, I started having panic attacks and felt frightened to leave the house. I still carry an attack alarm in case I meet them in the street. I'm dreading this Christmas, in the same way I dread every Christmas, because they will be alone for it and I know they are going to start on DH to 'make' me do as they say and let them visit.
BIL and SIL emigrated in part because SIL couldn't take the behaviour anymore and BIL wouldn't stand up to them in the way DH and I did. He actually left a fiancé years before, just before he met SIL, because she tried to stand up to MIL's controlling ways and he couldn't do it.
MIL was 47 when I met her, FIL was 49. They are both in their sixties now but even when we first met it was the fallback position in the family to excuse their behaviour as "thinking before they speak" and "too old to change now." I don't think so. By the time I met them MIL had already driven away her own brother and blamed her SIL for it.
They'd driven away most of their friends as well. They are very good at making new ones, very bad at keeping them for longer than a few months. They are nasty, spiteful, selfish, miserable, manipulative people and wether their cruelty to me was casual or deliberate, I tried for almost ten years to let it go and maintain a relationship with them before it became impossible.
If I ever do to my future DIL what my MIL has done to me and her other DIL, I wouldn't expect anything less of DS than that he cut me out too. I'd deserve it.