So this is all we have to go on speculations i thought you guys had more solid proof (not saying that your wrong) but who's to say that she didn't get the disease in skyrim and used her vampire seduction on the people and made some of them her thralls
Vampire seduction is not Volkihar exclusive. It was first used in Oblivion. Seridur used it to hypnotize Roland's lover to feed on her before she broke her neck. The fact that Harkon and his group aren't politically involved in mortal affairs gives us even more reason to believe Sybille was originally suppose to be a Cyrodiilic vampire. Janus Hassildor, an obvious member of the Order, sends the player to do his dirty work to exterminate a group of animals whom he considers base animals and savages(Cyrodiilic vampires are known to tremendously hate other vampires). He did it because he dislikes them, they are potentially dangerous to mortals in his territory and they draw needless attention. Same exact thing for Sybille Stentor. A woman of political office who is known to be a mage(probably to also help reinforce her masquerade as a vampire) is in no manner a Volkihar. It doesn't fit their nature to be involved in mortal politics.
Janus Hassildor also had a vampire in his castle that fed on prisoners rather than regular prey, considering they are horrible members of society that would not be missed. Sybille does the same exact thing as the Pale Lady, though the Pale Lady worked for Janus and Rona. I am sure none of them in Solitude are made thralls by Sybille. They aren't entranced or mesmerized. By all intents and purposes, she was merely blending in and influencing them through speech rather than supernatural power.