Ahhhhhh that's sweet of your DS
Ok if you can't manage to tell her to bugger off how about perfecting an icy stare and every time she mentions BMI fix her with it.
My boss is a health fanatic she was a size 8 when she had eldest and ended up with pre eclampsia and a prem baby and a size 10 when she had youngest and had GD and he got stuck on the way out so she nearly had an ELCS.
My mum was a size 10 when she had me, my shoulders got stuck and she bled out.
My Aunt was a size 10 with both my cousins and she had pre eclampsia both times.
All people who the NHS would have us believe are the ideal weight and all had problems, I on the other hand have a high BMI but have had a text book pregnancy so far (obviously don't know about birth yet).
Being a size 8/10/12 does not mean you are healthier than someone who is a 14/16/18/20 it just means you are smaller