The worst thing my MIL said to me was this: "Do you still know how to read, or do you just listen to books on tape?" and then she actually said "oops" This came about because my MIL was always passing on these stupid novels to me (not my husband) and I rarely read any of them. I do read but not as much as my husband or MIL. My husband probably reads 3 books a week. MIL was a teacher and reading was very important to her. At the time that she made that comment I was working and still raising kids and had very little time to read. I had to travel to a nearby town about an hour away regularly so I started listening to books on tape while I drove. MIL was always asking me what I was reading (even though often it wasn't anything) and I would feel like the bad pupil because I hadn't read any of the books she gave me. I just stood there with my mouth open when she said that though. I think it might be the meanest thing anyone ever said to me. ( I know I am not very thick-skinned)
I did confront her about that comment later along with talking to her about the passing on books to me and quizzing me about my reading habits. She laughed and said "Did I say that?" She never apologized or even admitted to saying it. That was probably about 15 years ago and I did talk to both of my inlaws about all of the issues I was having at the time. I was proud that I was able to assert myself at that time with them without losing my temper and they appeared to listen and didn't get mad. However nothing seemed to sink in... no acknowledgement or understanding or apology... just oh we love you, we never meant anything by it.
We went on for another 15 years before I tried to address the same issues again and I regret that I even tried because this last time they were completely defensive and turned everything I said back on me and then I completely lost it with them. All it did was hurt their feelings and cause me to behave in a way I'm not proud of. I wish I would have realized it was them and not me years ago and learned to just ignore them while protecting myself and saying a firm no to anything I didn't want to say yes to. But at this point since I have no obligation to my grown children or my husband to have a relationship with them, I don't see the point of traveling across country to have that kind of relationship.
It sounds like your MIL was more blatant. Mine tried very hard to be nice, but she blurted out these very insulting things at times and then other times she would gush with compliments. I think she tried very hard to say nice things to me, but it never seemed heartfelt to me. So it was hard to recognize that it was her and not me and hard to confront someone who was trying to be so nice. But her true feelings came out in subtle little sugar-coated ways that to tell you the truth I still haven't figured out. She is very confusing to me. She can be kind and other people in the family like her. I didn't take my FIL as personally even though he was worse than MIL in many ways but it was easy to see his behavior for what it was.