The choices that your sister gave are actually good for you! If you find an empire waist (which I think I see a similar one in the pic) in pastel you should be good! Chiffon is very forgiving! Depending where you go, I'd get it now because it takes a while for it to come in. I know at least at David's bridal that's the case. They normally suggest going 2 sizes up.
I belive David's bridal also has some styles in maternity. I agree that the dress looks like chiffon which is amazingly light for a summer wedding. It's what my bridesmaids wore (july 25 wedding) and no one was too hot.
Yikes! It's tough that she's having you find your own dress with such short notice. I second those that say to order right away. Most bridal shops take months to get in dresses. Also, I would go there and have them measure you and make recommendations on sizing if at all possible.
Re: I need a little help ladies