Not a credited fashion historian either, but I have spent a good part of my life researching fashion history.

Judging by the sleeve shapes, they look like modest "leg o' mutton" sleeves. They were fashionable in the 1890s. In 1870s to 1880s, such puffy sleeves werent fashionable quite yet. The woman in the center in particular looks like she's wearing a shirtwaist with a slight "pigeon breast" look that was fashionable in the late 1890s and 1900s. Idk what their social rank is, but it looks more like a practical version of the trendy silhouette or an early version of it.

You're right in that it is definitely not Civil War era. The skirts and bodices dont match the 1860s trends.

Unfortunately I cant tell what the object the man is holding is. Perhaps a pillow or a ball? And the boy in the back is probably not enslaved (maybe a servant/stable boy), but I'll leave that to someone more qualified to answer.

It's also important to note that old clothes were often updated to fit with the current trends and that working class women did want to look their best by trying to emulate the latest fashions, but they also had to be practical. Therefore, the silhouette isnt exactly what is shown in fashion plates, but the years can be estimated.

Photo of shirtwaists common in the 1890s https://petticoatsandpistols.com/wp-content/uploads/2014/06/Fanny-fashion.jpg

An example of an 1890s shirtwaist recreation https://blog.americanduchess.com/2020/04/history-is-cool-1890s-belle-epoque.html?m=1

Sleeves from the 1880s were much narrower https://www.uvm.edu/landscape/dating/clothing_and_hair/1880s_clothing_women.php

Edit: changed "a slave" to "enslaved" and added some links

I noticed the white boys dont look like they're a part of the household (roughed up, matching clothes, rolled up pants, barefoot, looks like both are holding hats for outdoor work). My intuition says they're maybe also servants and that the Black boy was pushed to the back due to his race (Black slaves could be seen painted "into the background" in old paintings as well). Could someone verify/counter this?