This might be a good opportunity to try "diplomatic honesty". It's hard to say, but if you say it honestly, perhaps feelings would be spared. I would tell the SIL that "While you're entitled to your own personal opinion of what is & isn't overweight, my family would appreciate it if you kept that opinion just that.....personal. It hurts my feelings and it hurts my child. If this request can be respected, then you are welcome to visit"......(I understand this one intimately because my beautiful 9 yr old has always been taller and bigger than her classmates).....take care.....grace