The figure on the left is s a picture of an evening dress with puffed sleeves from a 1889 fashion plate which illustrates quite nicely the elegant style of which one can easily imagine Irene and the Corey sisters as being representatives. The picture on the right is another depiction of evening dresses, this time from 1882. The similarities are quite obvious, however, especially in regards to the plunging necklines that are evident in both these illustrations. I would personally assume that the style chosen by the Corey sisters would more approximate the left-hand style, which is much more demure and sleek than the earlier styles. Both of these dresses would have been very suitable for a dinner party such as the one held by the Coreys. Both of these illustrations are taken from Handbook of English Costume in the 19th Century. C Willett and Phillis Cunningham. London, Faber and Faber Limited, 1959. 

In contrast, both of the pictures below (from Marquise de and and Fashion-Era) are representations of ball dresses of the era. As one can see, they,re a bit more ostentatious in design and decoration than the comparable dinner dresses, as well as being more revealing, both in the chest area and of the arms.