It's not impossible that they effectively make up their own "corps", but honestly, there are a
lot of levels that any militay specialist can be delineated (and there are distinction between operational, administrative and training "units" as it were).
First of all, I think we need to consider the Regiment-based nature of the Imperial Guard, which I would argue doesn't lend itself very well to specialized "corps", as each regiment is raised as an administratively discrete unit from the start. This means that if a sentinel corps exists, it would probably exist prior to the tithing of the regiment, on a planet-by-planet basis.
Secondly, I think we need to consider how sentinels and aeroplanes differ. Aeroplanes operate quite independently (even the fairly primitive pre-RF ones), and arguably excel due to the fact that they do not rely on combined arms operations (strategic scouting, deep-operative bombing, etc.) while they of course can be used to directly support ground troops as well. Comparatively, I think sentinels are a lot more limited in their "independent streak". Sentinels are fueled at the same depots, presumably with the same fuel, and they operate on largely the same plane and axes as other Guard (army) units.
That being said, Sentinels do offer a kind of mobility that few other Guard units have, and can as such operate in areas where tanks probably would have larger problems, and perhaps even infantry (a bit debatable, I'm not entirely sure of the fluff on this). Anyway, I think that these factors taken into consideration it's better to compare Sentinel-pilots to early modern or Napoleonic dragoons, in that they're sub-regimental units who retain an air of uniqueness and inward-oriented
esprit de corps (while not being a literal corps). You might have a full Sentinel Battalion, for example, or a Demi-Brigade/Demi-Regiment.
I don't think it's particularly unlikey that sentinel pilots are officers, but I'm questioning whether they require a rank as high as lieutenant. I think, perhaps that a sergeant or a second-lieutenant ("cornet" in British cavalry tradition, which I believe is a plausible comparison) is a better match, seeing as second-lieutenants would still keep the manpower to maintain each sentinel at an administrative squad-level (4-10 men), while still granting the pilots an air of prestige. The full leader of a squadron of Sentinels could then be a lieutenant or captain equivalent. I'm not familiar with Anglophone specific ranks for a cavalry captain, but Germanic languages (including my native Norwegian) have used "Rittmeister/Rittmester" as a term for a cavarly captain. Literally "Master of the Ride" (a rank used by the leader of the Cavalry Squadron which served as the recon unit of the mechanized infanty battalion I served in).
I hope some of this migh've been of some help.