Real Life MMD: What should I do about the bridemaid's weight?

My SIL asked her daughters and bro's daughters to be bridesmaids. Took them along to choose dresses several months before the wedding. One of bro's daughters is huge, I'd guess approaching size 30. SIL did best to find a dress that would fit and so forth and when my niece saw herself in it she burst into tears and refused to be a bridesmaid. SIL solved the prob by asking her to give a reading instead.

Niece is huge because she grazes all day and eats junk. It's entirely self-inflicted and she and everyone else in family knows it. Her head is so far up her behind in denial, that when her grandmother gently had a talk about weight and health, she got snarky and now refuses to talk to her grandmother at all, much to her distress.

I'd say that weddings are stressful enough without having fatties in the mix as bridesmaids and I say that as someone who was size 22 and is now size 14. I thought the reading idea was a really good one.