From what I gathered from my research, as DragonSlayr81 said, the original progenitor Belmont was Christopher. A couple of months later we got Ralph from CV3/Akumajo Densetsu who was orginally conceived to be same character as Christopher (see this
thread for more info). Later during the development of SotN, IGA created the official timeline for the series and he decided to split Ralph/Christopher into two seperate people, making Ralph the first one who killed Dracula and Christopher his descendant, who was in turn Simon's great-grandfather.
A couple of months after SotN got released in Japan, we learn about yet another progenitor Belmont character named Sonia in Konami's official magazine which has coverage about Legends and its story. The same issue also took the opportunity to publish the timeline IGA developed, which before that point had been for internal reference only. The timeline places her before Ralph, sometime during the 1400's. However, things changed pretty quickly afterwards, as the very next issue has a small note which states the timeline from the previous volume was "developed specifically for SotN" and is "unrelated to Legends".
The following is my speculation, as I don't have enough evidence for this claim, but my personal theory is that Legends was not developed with any of the other games in mind. I think it was supposed to be a different "world" from the one that IGA created. Therefore, I don't think Sonia was ever meant to be Trevor/Ralph's mother. So I suppose when IGA became producer, he took the opportunity to fix this mistake. Six years later, we got Leon, Trevor's ancestor in IGA's world.
...and I suppose this is all completely unrelated to the point I'm trying to make. The bottom line is that in both the original CV3/Akumajo Densetsu and IGA's official timeline which assimilated the events of that game, it has always been clear that Trevor was the one who defeated Dracula first.
This is so complicated to explain, I doubt anyone fully understood what I just wrote, haha.
But as I understand, Legends' backstory had some contradictory elements to what IGA wanted to keep sacred within the official canon. Dracula being Vlad Tepes was one thing, the bigger issue being Alucard's bloodline corrupting the Belmont bloodline that conflicted with LOI's story about their bloodline being pure. Otherwise the VK (with Sara's tainted soul) would have rejected another tainted soul. I wonder if IGA ever thought about keeping Legends but retconning the inference Trevor was Alucard's child.
This is actually quite amusing, as there's an interview from one of the SotN guides where in IGA's headcanon, the reason why Alucard can use both holy and dark weapons is because he inherited the ability to use dark powers from Dracula and the ability to use holy powers from Lisa, who he considers to be from a holy bloodline. It's the exact same situation as Trevor being Alucard and Sonia's child. Except everybody always took issue with it because of the bit described above. It goes to show that we fans don't really have any idea what we're talking about, despite claiming to be experts.