The nicest thing my MiL did - and she did this always, all the years I knew her - was never to interfere with how I raised the children, how I ran the household, how I cooked, or any of my lifestyle choices. She was such a lovely lady and I don't think I really appreciated how lucky I was to have such a mother-in-law. She said right from the outset that I need never worry that she would be the classic 'interfering mother-in-law', because she never would be. And she never was.
The only thing I would criticise her for was the way she favoured our DS over his sister. My DH was their only child and our DS was the image of him as a child, and I believe it was this that made them love him more. Recently, my DD has opened up about how upset she always felt when they stayed over for a few days, during half-term when I was working. It was so obvious to her that she was second best. Just little things, like praising everything DS did, but never her efforts, like being made to go to bed earlier because she was the younger one (I never made that difference as they were so close in age). She told me that she used to cry herself to sleep and pray that I would just turn up and take her home. I never knew
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