It's not a bad choice, it's a very classic dress. Considering some of the horrible BM dresses out there, I think you made a great choice. Maybe your sister is hoping you'll change it. And would that be such a terrible thing? She is the MOH, if it would make her happy, how about a different dress in black? Lots of people have the MOH in a different dress, either same dress different color, or same color different dress. According to the Ann Taylor website, they're getting new Celebrations dresses in early Feb, and they're still going to have black. Have you ordered the dresses yet? Maybe they'll have one she likes better. There is one other long dress now, but I think it's a different fabric. This is one time when the BM dress would be one she could wear again--people say that, but very few BM dresses look like anything but BM dresses, but a plain black dress is always wearable.
Personally, I think it's better to have happy attendants than a line of identically dressed girls. I've got three very different dresses in my bridal party, but they're all navy and tea length, so I think it will look good on the day.