I would try something like.

"I am not going to participate in this conversation. I will be reading in my room if you need me later."

or

"I will not allow all of you to berate me about my weight. I am going for a walk."

OMG, this sounds like when I was a little girl growing up! I not only had to hear comments from the kids at school, when I would come home to my family, I would hear them as well! NEVER from my mother, but from my bio. father and my uncles and my own grandfather.
So trust me, I feel your pain. All I can say is hold your head up and as hard as it is, DON'T let them get you down! If you do, then they win!
Remember, "He who is without sin, cast the first stone!"
Kudos to your son and remember you always have all of us!

((HUGE HUGS))

P.S. Usually people who berate others have a real problem with themselves and want to take the attention off of them by belittling others. We have a guy like that at work who loves to belittle others to take the attention off of himself. Just remember that when he does it next time and just LAUGH at him and say, "You must feel really bad about yourself".

Morrigan, I think the way you do it is the best! And definitely must be done with zero "attitude"

Oooooh this made me mad when I read it!!!! I'm sorry anyone has to deal with something like this, much less in your home, and with your mom joining in!!! Bless your son for speaking up for you...you've obviously raised him to be very aware of people's feelings!

My first reaction was to tell you to...well...I won't repeat it, but then I thought about the line 'I may be fat, but you're ugly and I can lose weight, what can you do?'.....might be a little 'nicer' to say, 'I'll agree to lose weight if you agree to work on how crappy you treat people.'

Good luck with your goals! You're doing great so far, and people like that don't deserve a second of your energy! You're winning a battle against him with every single ounce that comes off your body!!!