@JT: Twi's parents are likely to be civil servants or lower-ranked nobility, possibly with property but not land (Or at least not more land than their possible mansion). Birth rank, minor noble.
She's the Chosen Faithful Student of Princess Celestia, so she's also a Royal Scholar and Celestia's protege. Notice that the guards never seem to be bothered by her presence in supposedly high-security areas of the Castle and Library, and Celestia was all too happy to let out a room to Rarity and even give access to the large Ballroom for various birthday celebrations.
Her brother is essentially Captain of the Guard and married to a Princess, so she's in-law to Cadence (elevated to higher nobility by marriage). The guards are probably sometimes thinking "Oh, it's Captain Shiny's little sister. She here to meet him today?"
As of the S3 pilot, Cadence will be undertaking a campaign to reclaim her rightful ancestral throne of the Crystal Kingdom, so Twi's rank will rise even further, especially if Cadence personally requests her to help. (Consultant/Historian for the Crystal Kingdom? If Dashie helps out just to fill her Daring Do fangirling, she might also become a Royal Archeologist for the Crystal Kingdom :D)
Rarity might get jelly, but she's probably happy enough being best friends with Twi and she's already a Royal Dressmaker (having made Cadence's wedding dress).
Furthermore, as bearers of the Elements of Harmony, the Mane Six in general are already titled and have pretty high security clearance when it comes to dangers to Equestria.
edited 16th Aug '12 7:54:41 AM by AceOfScarabs