I'm thinking of joking that it would be healthier for my brother's small-chested overweight fiancee to just lose weight instead of getting a boob job after she said she wanted a boob job to be proportional
That sounds kind of mean? How do you think she can get healthier?
I think she needs to lose weight asap.
It is okay to have an opinion but you shouldn't joke about her body like that. It will upset your brother and it could ruin your relationship.
I want to see my brother in pain. I'm going to post this on his wedding Facebook.
Why would you want that? Your brother is your family and he loves you very much, you should feel the same way back. Saying something so crass about his fiancee is sure to ruin your relationship with the both of them.