According to TV Tropes, under the Umineko Characters Page: Ambiguous Gender: Very intentionally; The narration refers to them very gender-neutrally, they have an androgynous appearance, and nobody seems to know what their gender actually is.
Prepare your butt, boy!!It suits you, Diana.Depending on the circumstances, you could also have ended up as a servant.Oh... really...But, if Will was my master...Maybe I wouldn't mind being a maid~