Yeah, I don't think Bruder and Schwester would be so bad either. And if it's in the context of a Bruderschaft, it shouldn't make readers wonder too much either.
The question is: Does the average German reader know the difference?

Or would they even consider it a difference in contemporary German? To me, personally, it sounds pretty much like the same overall concept (like, some groups joining together for some cause?) at least. In fact, I couldn't tell you what the difference is exactly if you asked me right now. There might be a historical one or maybe in a political sense but I don't think it'd make that much of a difference in the story itself.
Other than maybe Fraktion. That does give me a different feeling even though it's still about groups joining together
I think that totally depends on what exactly you write and how you go about it. If it's just xianxia and just for the fun of it, then yeah, writing in English and publishing here is likely better. When considering avenues to sell though, I have made the experience that I'm actually making more bank with German stories on Amazon than with the English equivalents. It usually can't compare with Patreon but it's still nice to have on top if you want to live off of writing. So translating this is kind of important for me. I was just dragging my feet on the cultivation stuff because it's more difficult than the others so I started on those first. But there isn't that much other stuff left so I can't keep procrastinating forever