However, Kongou's sisters all call Kongou "onee-chan". We never ever hear Mutsu refer to Nagato as her "onee-san" or "onee-chan", nor does Nagato ever refer to Mutsu as her imouto.
So while Hiei, Haruna, and Kirishima are all a bunch of siscons, I don't think we can really apply that label (or the incest label) to Mutsu and Nagato.
Roosterteeth has sold specifically bumblebee ship merch and their VAs and plenty of staff openly ship them yet the most official shit is yang liking blakes new hair. As for ruby and Weiss they have even less official sustenance but maybe Weiss’ mom has a chance
^In my experience half the time they do that they don't actually understand the first thing about the relevant topic(s) and the result is worse than just handwaving it... "Artists Are Not Engineers" as it were, and most seem quite lost on even something so basic as how ectothermic ("cold-blooded") metabolism functions in practice. (You run into that an awful lot with reptilian-themed critters because obviously scales = ectotherm amirite guise?)