You know, I'd like to put it just a little later, say 1888 or even 1889...it's because of the split look to the bodice, and the fact that there seems to be a very slight kickup to the sleeves, a fashion that got its big start in 1889...just my opinion.
The similarity in dresses may just be that the material was being ordered, and the style of dress commissioned...all clothing was made by dressmakers then so it may have been as easy to make two...I have seen in other photos sisters wearing very similar dresses.
Cheers,
China