It's OK.
It would need heavy reworking for it to be more plausable.
The part I like is the one about a gifted child born with powers from a long-line of warriors (Classic Castlevania right there), thus that person is extra strong.
I also like how the weapon origins came from that game (no longer true, but in the game you had to collect magical artifacts that would become the standard sub-weapons later).
Lastly, while I despise the bad platforming and enemy placement at times (bad usage of RAM, Konami of Nagoya, BAD!) I do not mind the castle progression, what with it going linearly as opposed to the MegaMan-style stage layout of the previous GB title.
A lot of people cannot fathom Alucard and Sonia getting together... but no one seems to mind that Dracula and Lisa got together. They also cannot imagine the idea of the Belmonts having a little 'streak of evil bloodline' in them, but I think that, considering all the possessions that seem to happen later down the bloodline, it is quite a possibility.
Soleiyu seems to easily be corrupted, and later on the same thing happens to Richter. If they're so strong and powerful, something like a little mind manipulation should not affect them. I think it gives them an Achilles Heel, personally.
I'm opening a big can of worms with this reply, but maybe it'll spark some good discussion for this hot summer for people to ponder while in front of their nice Air Conditioned rooms.