^it's not, but it seems to be loosely based on the gaudy uniforms of the period. More specifically the transverse braiding characteristic of the infamously flamboyant hussar uniforms, though the artist seems to be unclear on the details and has made them buttoned flaps instead. Compare the (for the day) understated Húsares de la Muerte uniform (Chile, late 1810s).
Minus points for not showing off Dat Ass with the very tight-fitting breeches nigh-ubiquitous in the uniforms of "true" cavalry (as opposed to dragoons who were technically mounted infantry) those days. :P
^ I think is a reference of how often Fate is portrayed as "trying" to be Top... only for things like this image to happen.
Think Matsuri and Luna from Hololife.
This image becomes so much hilarious now that we canonically know that Saki is short. She just subtly hovers a foot into the air and slides over to Yachie to assert dominance. Absolute queen shit.
Breaking fragile things is more Flan's purview, but I suppose Remilia's probably too strong for her own good as well. It feels rather ironic to see her tending to Sakuya, but I suppose it's also strangely fitting, especially if you subscribe to the theory that she pulled Sakuya out of a dark place and altered her tragic fate.