I think people are getting Isabel calling Levi “aniki” mixed up with her calling him “oniichan”, which is not a thing she does and this is actually very important for a couple reasons:
1. because “aniki” also carries the connotation of yakuza, where younger members call the higher ups aniki. It still means older brother, but has a rougher meaning and is used almost exclusively by males, which brings me to my next point,
2. isabel refers to herself using “ore”, which is also another almost exclusively male word.
if im not mistaken (im too lazy to actually go find it again) farlan also calls levi “aniki”. this furthers the thought that they consider themselves family (like yakuza do) but not in the literal blood related way
anyway i think this is important info to know and now you’re informed!