Board Thread:Questions and Answers/@comment-25951143-20150101110046/@comment-26269946-20150402122017

Alluka was a cis male however Nanika may be female and Alluka doesn't want to make Nanika feel uncomfortable so she likely transitioned to be a girl. Killua says that Alluka is his little SISTER multiple times (the manga misgenders her). The Zoldyck family misgenders her because they're asses and they don't like Nanika and so they disrespect her.