Her character also points to her being part of the Order. She looks down on the other vampires and considers them little better than beasts, which fits with the Cyrodiilic strain's disdain for other vampire breeds.
Sure it's all conjecture and speculation, but a lot of the observable evidence points to her being a member of the Order.
You guys know I normally agree with you but I can't here. Let me explain why. Some of you seem to believe that only the Cyrodiilic vampires blend into society, likely from the book Immortal Blood. Some say their ability to blend in in-game, breaks lore. Regarding vampire lords breaking lore. Consider this: vampires in the world of Nirn are shrouded in myth, folklore, and half truths. The majority of npcs in oblivion don't think vampires can EVER see the sunlight. In the book "the locked room" the author apperently believes that vampires pass out once the sun rises, only to awaken when it sets. So you see, there's a lot of falsehoods and misconceptions and simple unknowns about vampires. Whose to say that volkihar blending in was just largely unknown ane scarcely written about. They seem to be very clever and powerful and likely hid this ability, only revealing themselves as vampires in their monstrous form. The Cyrodiilic vampires aren't the only ones who can take human form. If you're well versed in the lore you'd know that some vampire tribes are shapeshifters / bodysnatchers and take the form of children they feed on, only to live in that person's family, waiting for a moment to strike. Long story short, it's easy to find these guys a place in TES lore.
Now, keeping that in mind, we have Sybille. Dont get me wrong, I DONT like her appearance, but I dont think she's a Cyrodiilic vamp, nor intended to be. We know that the volkihar have spies in cities, and we know that they have their own political structure. Is it really hard to believe that someone from that background wouldn't hide within a skyrim court?
Vampires in general shouldn't have been so obvious when they're "blending in". Blending in is the whole point. If it were just orange eyes it would be fine: something that's a hint to vampire hunters but not solid proof for the public. But the glowing eyes are a dead giveaway. I can't suspend my disbelief and pretend that no one notices that. Even fangs can be hard to see sometimes if you focus on not opening your mouth too much, not allowing more than a passing glimpse of them, but glowing orange eyes are unavoidably noticeable as long as your eyes are open.
I'm pretty sure you guys will still disagree with me which is fine, but I just wanted to throw in my two cents.