It really depends a lot. Some people only put on 6 kilos for their entire pregnancy and others put on 36!

What style dress is it, and if you had to get a different one, would your sister be devestated and could you get a style that would be suitable (i.e. empire line). This is much easier if there is only 1 bridesmaid, or more than 2, as you can easily have different styles of dress in the same fabric or colour.