Emily's bridesmaids' dresses looked great. From what I could tell from her pictures, none of her girls looked awkward or strange in their dress. Since I had just one attendant, I told her to wear whatever she wants. Its ironic that both Emily's girls and mine wore black!
The problem w/ my friends bridesmaids' dresses was just about everything!
She chose maroon. They were strapless. The skirt was big and poofy w/ that thule stuff underneath it. They were very HOT (as in warm, not sexy). I felt like a goon wearing it. I don't recall what I paid for it but it has never seen the light of day since that warm August afternoon 5+ years ago.
For an outdoor wedding, I think tea length is ideal. While I would have loved a longer dress for myself, it would have been dirty in about 5 minutes. It simply was not practical.
As for your sister, maid of honor, wearing white along w/ you ... I don't know about that. I think it could be fine if she's set apart from the other bridesmaids (e.g. different dress, different color, etc...), but she should not be set apart in white. That's reserved for you, assuming you're going white.