You're in the mafia now, n'wah!
The Telvanni are a Dunmer noble house, the only one of those that didn't join the "Pact of the pitiful". You can compare the setting to a mafia family, the Archmagister (in this case Vivicah) is the Don(a), and the councilors are her "cousins". Since Telvanni is a noble house, all the Telvanni nobles would in some way be related, more or less.
Like mafia, Telvanni can make you disappear. Like mafia, the authorities won't question too precisely, unless someone really important disappeared, which is why the Telvanni, like mafia, won't kill anyone really important. On that note, an interesting tidbit of history from the first Era, the Armistice.
Morrowind was never invaded. Whenever someone got too close to that, the ruler-in-charge had a terrible accident, like it happened to poor Versidue Shaie in 2E324. The empire is in disarray ever since, and that was because he pissed the Dunmer off.
Lesson from that: Do not piss the Dunmer off.
Like mafia, as long as you talk to the lesser ranks and not to the Don, you're good.
The Telvanni have been known to be the most "open-minded" of all Dunmeri noble houses, as in, they are neither racistic nor speciesistic and instead just hate everyone equally. Non-Dunmer can become House Members on their own right, and they will not be treated any differently from Dunmeri Telvanni: Entirely based on their level of competence, skill with magick, and usefulness.
Dunmer politics are very subtle. The Dunmer have perfected the art of insulting and threatening each other by not insulting and threatening each other. Outlanders negotiating with Dunmer should be very aware of the fact that as soon as the name of any other house is mentioned, they have entered Team Deathmatch as the only one unarmed.
In addition, threats that are very subtle by the standards of other races are probably rather aggressive from a Dunmeri point of view. We had a rather ugly situation the other day where a statement like "wouldn't it be terrible if the Indoril heard something bad about you? By the way, I'm a trustworthy source! But I won't tell them anything!" almost led to an on-the-spot execution because the Telvanni didn't appreciate having a knife at their throats.
Beron Telvanni will act as a steward for the Nobles, starting today, so if you ever feel the need to see some Telvanni, he can give you an in-character briefing, or you can tell @manavortex, @ScattyThePirate / @ShanksThePirate or @DaedricAdept for more input.