Question about attending a wedding

My sister in law is getting married in October and she wants me to wear either a burgundy & black dress. I will be around 30 weeks around that time. It's a mid morning wedding and I don't know if I should wear a long or short dress. She is telling me she doesn't care but she is very picky so I'm on the fence about what to get. Also, where are the best places to shop for a dress?