It was annoying when we were together and the dc were small. Mil was obsessed with ds1's feeding and kept telling us to 'ration' him to stop him feeding at night otherwise he would get fat 
. When they were toddlers mil and sil and others were particularly concerned about the amount of grapes and raisons they ate, tutting and clucking and commenting on the amount of sugar in them. I know they are high in sugar, but the dc never ate silly amounts of them and the comments were irritating.
One memorable Christmas when ds1 was about 2 someone commented on how cute he looked in his outfit and mil announced 'Yes, fat children always look better in their clothes.' He had normal puppy fat for a 2 year old. I thought it was a horrible comment. He was a good eater and was always referred to as a gannet by ex's family whenever he ate anything at all. On one occasion when he was about 4 he ate a smallish portion of a roast dinner and sil was disappointed and said she 'would have thought he'd be eating like an adult now.' Can't win.
Now ds2 has just come back from a visit with ex and family and has been a bit quiet. When I was putting him to bed he said 'I look bad don't I?' It turns out sil has commented a few times this weekend on how skinny he is. 
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Both dc are a healthy weight for their heights (afaik - ds2 hasn't been weighed or measured for ages, but he's growing, has lots of energy and you can see his ribs, which I understand is normal). DS1 (11) was just measured at school and, as the second tallest in his class, was on the 7th centile for BMI, so I guess his nightly bf'ing, raison addiction and fatness as a 2 year old haven't led to the obesity mil predicted.
AIBU to wish these women would stfu about my children's eating and weight as they know fuck all about it and wibu to ask ex to pass that on to them?