Wow, I love this idea! I have far too many letters to write but I'll save you all the earache and just do the one
Dear mother in law,
Your son loves and married a fatty and no amount of comments you make either to me or others will change that. If you love him, you should learn to accept that and keep your bigoted nose out of it.
I will lose weight and I will do it for me, not because of anything you have said.
Oh, and the next time you tell me I can't have any homemade cake because I'm on a diet, said cake will be shoved in your face.
Lots of love,
Me
Man that felt good!! The cake thing - true story.