The Sentinel Raven should probably not be a valid target for Awaken.
1. It's arguably a valid target, RAW
The Awaken spell states that the caster must:
touch a Huge or smaller beast or plant. SRD p. 120
The Sentinel Raven is a 'spirit sent by the Raven Queen to watch over you'. That 'spirit' 'assumes the form and game statistics of a raven' (Unearthed Arcana: Warlock & Wizard p. 2).
However, unlike familiars provided by the Find Familiar spell there is no explicit statement that the Sentinel Raven is not a beast. So it could be inferred that there is a loophole here that, RAW, allows the casting of Awaken on the Sentinel Raven.
When something takes on the game statistics of something else, that is normally considered to include the creature's entire stat block, bar anything that's been explicitly excluded. In this case that would include the Raven's status as a 'Tiny beast' (SRD. p.387) making it a valid target for Awaken.
2. However, I wouldn't allow it in my game
For the reasons you've stated, and probably others, it would be extremely beneficial to be able to cast Awaken on a Sentinel Raven.
However, Unearthed Arcana (where this archetype comes from) is playtest material and has been released for people to use without having been fully balanced for normal play.
It's likely that the Sentinel Raven has not been deliberately omitted from the usual prohibition that deliberately prevents familiars being appropriate targets for Awaken. This is the kind of oversight that's often removed before publication.
So, allowing Awaken to be cast on a Sentinel Raven in your game, while arguably not against RAW, could be a major source of imbalance.
It would certainly make Raven much more powerful than a normal familiar and so this loophole has arguably not been created deliberately. This is how I would rule it at my table, but you (or your DM) are welcome to rule as you wish at your own table.
