After the way my own MIL treated me,straight after marriage its a wonder that a) I'm still married to her son b) have a relationship with her that goes beyond basic courtesy.
She was beyond horrid, downright cruel most of the time. Never thought I was good enough for her family. Amongst the numerous things she said/did the following particularly stick out:
Telling me to lose weight before the wedding so that I wouldnt embarrass her family ( I am 5ft 6' and was a size 12 at the time)
Telling me that I should not expect to look good in the wedding photos as I was an old bride ( I was in my late 20's)
Going through all of our wedding gifts and cherry picking what she was going to have (tradition apparently-south east asian MIL), she also took the money we were given as bride and groom totalling almost a couple of grand.
Telling me that I should be grateful that her son married me because generally speaking my family and I were not really the kind of people she and her family would associate with ( this when I was 3 months pregnant and having a lot of problems/bleeding )
She cried throughout our religious ceremony and most of the day, every single picture she is in she has a face like a slapped arse.
Insisted on naming DC1 even though she was out of the country when Dc was born, leaving me to cook, clean and look after FIL and her home at 8 months pregnant.
There are many, many, other things she has said and done over the years. DH was useless in dealing with her partly because he was in denial as to exactly how horrid she was and partly because whenever he stuck up for me his whole family would give him a hard time for "upsetting mum".
MIL has calmed down since those nasty early years, I've learnt to pick my battles, I don't put up with her shit anymore so will stick up for myself if she overstep's boundaries but mainly I try and detach from her and her overbearing family as best I can.
She is good with our DC but still makes snidey comments about how DC looks like me as opposed to her son. Also refuses to attribute anything positive DC does to me or my parenting, anything good behaviour or intellect wise is down to her side of the family and their superior genes.
I think with some MIL's there is literally nothing you can do to maintain a good relationship. For some MIL's the mere existance of DIL is enough to send them batshit.
Excuse length and also typos.