I get on ok with my MIL. It has taken nearly ten years to get to this point, and over a year of therapy on my part.
She is not an easy woman to get on with, but I am not blameless at all in the deterioration of the relationship.
My husband told me such terrible stories of his childhood when we first got together - the pressure, the love that was conditional (he only felt worthy if he was doing well at school etc), the beatings, the favouritism towards his sister etc etc - that I had formed a poor opinion of them before I'd really met them properly. Their behaviour in the first couple of years of our relationship backed this up - neediness, no understanding of space, demanding etc. If ever he wasn't free to go round when she wanted him to, she wanted a blow by blow account of what we were doing, where we were going etc. It was invasive, and totally unlike my family and I couldn't cope.
We muddled along for a few years, then had DS1 - the first few months were horrendous. Demanding to see him, making out like she couldn't get through a week without seeing him (I thought once a week was reasonable, apparently I was wrong), then insisting that she have him when I went back to work. We agreed to this as I thought if she had him in the week she might not mind so much if we didn't go round every weekend - fat chance. Anyway, I had decided to take a year off for maternity, but she hassled me from the moment he was 6 months and I ended up going back to work earlier than I wanted to, just to get her off my back.
Anyway, 5 years later and a lot of work on my part, not least recognising that she wouldn't change so I had to, (thanks MN!) we have a relationship where I don't want to rip my own eyeballs out in anger every time I see her. We can chat and laugh. It's very superficial (on my part at least), I don't consider her a friend, she is not someone I would ever go to for help for myself, not someone I could ever confide in or anything like that. But we can have a cuppa and a natter about the kids. If nothing else, we both love my boys, so we make it work. I don't go round at the weekends anymore, my husband takes the kids and I get the time to myself. I consider the relationship low contact - although I see her anything from once a week to three times a week depending on school holidays, it is for a couple of minutes when I drop the kids off.
I don't hate her. I don't love her. I cope with her and it works. I feel better about the whole thing and my husband is no longer stuck in the middle. I was on the verge of despising him for his lack of a spine - he once told me that while he agreed with everything I said about her, he would still rather upset me than her because I would forgive him later. I told him in no uncertain terms that that was not the case and that if that was how he saw it then I was off! But I did see that he was torn and he is still a frightened child desperately seeking his mother's approval, which he has never had. No matter how hard he worked at school or in life, he has never been good enough and probably never will be to her.
My SIL is another matter though. The golden child, could do no wrong etc etc, allowed to bully and hurt my husband when they were growing up with no comeback. She is now unable to hold a proper conversation - she literally just makes grunting noises when you speak to her unless you really manage to get her on her own for a length of time. She is massively intelligent but can't do her own washing. It is scary to see. I have tried to help in the limited way I can but I don't think there is anything I can do. She gets on very well with the boys because they don't expect an adult relationship - I think it says a lot that she gets on better with a 5 and 2 year old than anyone her own age. It's sad really but I refuse to let it be my problem.
Sorry for the ramble. As you were.