Could her Norman ancestor have been a mercenary knight or man-at-arms? The Normans certainly got around and there were enough wars going on in that part of the Balkans. I think it might be profitable to have a look at the 4th crusade that took Constantinople. Many French/Norman knights took part in this as did the Kingdom of Hungary.
According to "Castlevania: Lament of Innocence" for the PS2, in 1094, Leon Belmont was a former baron that goes to rescue his fiancee, Sara Trantoul, from the vampire Walter Berhard. Leon rescues Sara, but she slowly becomes a vampire. Honoring Sara's request, she is sacrificed, and her soul is infused into Leon's whip, which is henceforth known as the Vampire Killer, the Belmont family's ancestral whip. Leon goes and defeats Walter, but it's revealed that his close friend. Mathias Cronqvist, orchestrated the entire thing in order to gain immortality (albeit as a vampire) in order to curse God for taking away his beloved wife. Leon vows the Belmonts will hunt down Mathias, who is revealed in the epilogue to become Dracula.
Now, that's all backstory. Legends (and thus my fan film) occurs 356 years later (there are no games that occur between the two). What we know of Sonia Belmont is she was born with special powers in a remote area of Transylvania, and she was constantly told while growing up that her powers were meant for a greater purpose. She's trained to use a whip by her grandfather. In 1450, at 17 (or in her seventeenth year), she meets Adrian Fahrenheit Tepes, also known as Alucard, Dracula's son. He's trying to stop Dracula from destroying humanity. The game suggests Alucard also trains Sonia and that they're perhaps lovers (although it's vague). In the "best ending" epilogue, it's revealed that, sometime later, Sonia has a child that will also have an adventure. Due to the way that it's worded, it's seemingly implied that this child is Trevor Belmont, the hero of "Castlevania III: Dracula's Curse" for the NES (which occurs in 1476, 26 years later), and Alucard is supposedly his father (and thus Dracula is his grandfather), but, again, it's vague.
Legends was removed from the series' official canon some years later by the series' then-producer, Koji Igarashi. It's been assumed by fans that he removed it due to it seemingly conflicting with the backstories in Dracula's Curse and Symphony of the Night (supposedly, Dracula began his war against humanity only in revenge for his second wife, Lisa (Alucard's mother), being tried for witchcraft and burned at the stake, which was assigned a year conveniently after Legends' 1450, but I honestly have not yet found out where the specific years mentioned in any of the games came from, since no specific years are mentioned in the early Japanese instruction manuals), although the only reason that Igarashi ever seemed to give was purely sexist.
So, yeah, I'm working from a "noncanonical" game (although, since the original universe seems to have been abandoned, at least temporarily, in order to let a Spanish company make a reboot series, the first game of which is already out, I suppose worrying about the game's canonical status is moot).