hello!! mizusaba's gender is not indicated in the japanese text. they're never referred to with any gendered terms, and they also don't use any gendered terms about themself either. this is what we know:

• mizusaba speaks similarly to ibuki (using wa~ as a first-person pronoun and "sa~" or tildes at the end of their sentences)

• at one point, kanata compares mizusaba to madara (when mizusaba explains their presence in konata's life, kanata says it's "like what mikejima was to me in the past")

• when kanata greets mizusaba in okinawan by saying "haitai", mizusaba replies back with the same greeting, then informs kanata that he doesn't have to speak okinawan with mizusaba. "haitai" is the greeting used by okinawan women, whereas "haisai" is the greeting used by okinawan men. since kanata was first to use the greeting, it's not clear if this was an error on the writer's part or not

all in all, unlike other prominent NPCs that were introduced in climax stories, there's no true clear indicator of their gender. JP fandom noticed the ambiguity, and have been debating about mizusaba's gender based on the information we have. this is likely why you're seeing "brother" and "sister", because technically, it's all up in the air right now

at first, my tl used he/him pronouns for mizusaba because of the madara comparison, but then i realized it was entirely assumptive on my part, so i've changed to they/them so that people are free to interpret mizusaba as they like. the JP fandom have been doing the same and interpreting however best suits their vision of mizusaba. i hope this helps clear it up!