The clan name is also known as "Fernandez", perhaps the original intention of the name, and "Belnades" could be a mistaken transliteration, because:
Modern Japanese lacks the Old Japanese syllable fe /ɸe/, so Sypha's creator must use the closest-sounding native syllable, be べ.
In Spanish dialects, "z" is either pronounced /s/ "s" or /θ/ "th", and both are realized as "s" by Japanese speakers (cf. English health > herusu ヘルス).
The sounds of "-el" and "-er" are usually translated into "-エル" (-eru) in Japanese.
Since Trevor Belmont and Sypha Belnades are referred as a married couple after the events in Castlevania III: Dracula's Curse, it is possible that they had two children: one to continue the Belmont Clan, and the other to continue to Belnades Clan. If this theory is proven to be true, it can be explained why Juste Belmont is descended from both the Belmont and Belnades clans.