I am with OP. If anyone, DD, DDiL, DH or anyone else, started trying to manage me in any way, they would soon get told when to step back, gently and kindly if it was DDiL. A bit more briskly if it was DH or DD.
I think the OP within her own family is capable of knowing when her DiL is acting in away to control what she eats, particularly as DiL is constantly commenting on OP's weight 'problem', as DiL sees it. You do not notice portion size in your family Trisher anymore than I do in mine, but that is because it isn't an issue so there has been no need to watch how meals are being conducted.
I do not think the fact that someone cares about you is a justification for them to try to control any aspect of your life or try to force you into a course of action you do not wish to take.