I was massive already by 20 weeks...I'd already put on about 10 kgs (I started at 65 kgs and have now put on almost 15 kgs!) and my tummy was quite big (well, I thought so anyway). But I have friends who hardly had any belly at 20 weeks...it all depends on the person.

However, I got married last September and one of my two bridesmaids was 6 months pregnant at the time...it was hard to choose a dress size for her because we weren't sure how much bigger she'd get. We ended up getting one 4 (!!) sizes bigger than normal and it only just fit her on the day...however he was an IVF baby and she had gained a lot of weight due to fluids. The dresses we had were tight around the boobs but flowing from under them...luckily because a fitted dress wouldn't have worked for her.

I think it would be worth having another option 'just in case' but I'm sure your sister will understand and not worry too much...afterall, pregnancy size is all so unpredictable... Good luck!!!!