I look after my DN regularly. He is 6. He has a health condition which has the potential to be made worse by being heavier. The other a day I had to weigh and measure one of my DC for a form. All the DC joined in (they like it being plotted on the wall). As DN was here he also joined in. Whenever I weigh and measure them I also put the figures in the NHS BMI calculator. DN is significantly chunkier than my DC but the calculator put him in the overweight category. To me he visually looks overweight too - it's not his build. He is chubby. Obviously this is a sensitive issue but as I said previously he has a health condition which won't be helped by being too heavy (joint based). I haven't said anything to my sil and I won't but I did mention it to my MIL on the premise that as a family we could work together to encourage them to be more active. She was very defensive, suggested that he wasn't overweight at all and just a different build to my DC. I did say to her that I wasn't being personal and it was an objective thing to say but I think I offended her.
WIBU unreasonable to have mentioned this to MIL?