Dating the photo merely by fashion, I'd say probably early 1870s because it seems that the bustles are rather large, and the overall frilly-ness in trim and slightly looser fit of the bodices indicate the early bustle era, probably between 1870-1875. I could be wrong, though, & in that case it would be 1884-1889, the 2nd large bustle era, but it doesn't seem to be that to me...
Oh, I see that you've posted a date, Svetabel! Knowing that you're far more knowledgeable about Marie Feodorovna than I am, your dating is probably more accurate, though Queen Olga's dress does not seem to be very natural form era to me, because of the jutting out of the bustle.