Kalluto and Kurapika are confirmed to be male. Alluka and Neferpitou, on the other hand, are not yet confirmed.
Alluka is referred by Illumi and Milluki as their brother while Killua calls Alluka his sister.
Neferpitou is identified as male in Hunter × Hunter Character and Word Data Book and is shown to have flat chest. Later in the manga, Neferpitou is described like a mother.
It is possible to that Neferpitou's gender in the manga differs from the anime.