Well we all know how Charlotte finds the spell; Inside one of the paintings they were traversing. The Egyptian painting if memory serves. The only one who knew about the spell or at least heard of such a spell was Wind (Eric Lecarde). But he did not know where it could be located. Are you by chance having Charlotte be a relative of Shanoa's? Remember though, Charlotte doesn't have any of the Belmont's power within her or it would have been mentioned in her PoR backstory. That and she's not mentioned to be related to the Belnades family either. Shanoa does have the Belmont power, but it was an unnatural occurrence; a blessing in disguise from absorbing Dominious, and probably cannot be passed down to the next of kin as it does with the Belmont's natural-born power. It'll be interesting to see what you can come up with
Charlotte
does have a connection to the Belnades, but IGA never elabored on that, even though he did mention it in his interview about Portrait of Ruin. Charlotte will be the great-aunt of Lenora Belnades in this story, and have nothing to do with the Belmonts through blood. Her family is to the Belnades what the Morris are to the Belmonts, probably.
And no, Charlotte is not a relative of Shanoa. All Shanoa did was fabricate Sanctuary from the Belmont power she got from the villagers. As soon as she created the glyph, this power left her body to work as the glyph itself.
And Wind, apparently, didn't even know something like that was possible:
Charlotte: I wonder if there's a chance we can save those two if
I use some kind of purification spell.
Eric: The chances are very slim. You should know that. Even I
never knew how to do it. I appreciate your sentiments, but
I am resigned to their fate.
Neither him, nor Charlotte... The most we can see here is that
just maybe Wind thinks a purification spell could work but not even he knew how it could work...
...but Vincent Dorin
did have a vague knowledge about how purification spells work, possibly because he's an insider from the Church. You can check that in the dialogue he gives while being vampirized. More on that later.
Here's somewhat of a hint about how this "spell" landed in Charlotte's hands:
You're right about it being hidden inside one of Brauner's paintings. Now, what are his paintings? Why would a vampirism-nullifying magic be inside one of his creations? Are the worlds inside the paintings real, or are they created from something else?
The answer to this involves a very vague line from Wind in Portrait of Ruin concerning Brauner's existence:
Wind: It is indeed Dracula's Castle. However, the castle's
lord is a vampire by the name of Brauner.
Charlotte: I've heard nothing about that!
Wind: As one would expect. His identity has been kept secret
for years, after all. He infuses magic into paintings to
increase his power. You will undoubtedly come across
these paintings in this castle. Search for these paintings.
Why would this be? What happened to Brauner that became a secret that not even the people hunting Dracula could know? And one more hint. Remember the Vincent dialogue I mentioned up there? This is the one:
Charlotte: J-Just calm down! You've still got time before
you turn into a vampire.
Vincent: !! Y-Y-Yeah... That's what they say. Purification spells
are effective in the early stages. I beg of you! Do
something for me! Get that purification spell!
Now keep in mind: Charlotte finds this spell inside a painting made by a vampire, and the knowledge about it was vague to null even to her, who is a magical genius. But Vincent somehow heard about something like a "purification spell being effective against vampirism in early stages" from someone.
So, I
think this might help you piece what I have in mind together