I'm sorry. I wish I could say I haven't BTDT but I have with my Dad too. I know how frustrating it is!
It got better when I decided I'd just get up and go home any time my weight was brought up. It isn't up for discussion, period.
I remember in the heydey of laying the boundaries I once told my dad "Look, I have PCOS, I'm obese, it sucks, I try to improve as best I can. But you are being rude, and I don't see where you even bother to try to improve that."
So a few times of my just calmly saying "Ok, thanks for the visit!" and just going home solved the bulk of it. If they want to talk about PCOS in an educated way, fine. But I'm not up for being the freak show for dinner entertainment, and I have my own home to be at, so I go. :P
They all know that I'm just going to get up and go home if I feel like it's crossed a line. They've gotten better and actually tried to educate themselves more about the condition.
Two of my other cousins have turned up with it so that also makes it a bit easier -- I'm not the only one any more! It also makes it easier that time has passed.
I think for Dad it also was tied up in the mental adjustment of "These are my kids, so I'm in charge and all in the kids business because I AM THE DAD!" While I didn't get the dx til I was in my mid 20's, there'd been "something wrong" since I started menses at 13. So through part of this PCOS journey both my parents were knee deep in my health business because I was their child.
He had to learn that his role is now "I'm the dad to two adult women, and I'll always be the dad, but some of their stuff is their private stuff and I don't go there. It isn't my job because they are no longer my dependents."
I think that shift was easier with my sister because lucky me... I was the oldest and had to transition to "Adult Daughter" first.
A.