IANAL and all that, but really, there’s not enough info in the scenario you outlined for any kind of definitive answer, and, as has been pointed out, since you’re in a fantasy universe, whatever works best for your story will be the best answer.
However, in the late 20th/early 21st century U.S. Army, there’s a few variables in play here. As Tripler pointed out above, there’s a couple of articles in the UCMJ that could very easily apply in this situation. IN a perfect world, the Sergeant’s and Private’s testimony, coupled with the civilians, could see the Lieutenant relieved for cause, and court martialed out of the service with a Dishonorable Discharge.
In our really real world, some other factors could come into play, like the W.P.P.A. (West Point Protection Association), and oft-derided yet no less real “clique” that protects West Point grads at almost any cost.
Then there’s a small yet vocal subset of officer who believe (once you strip all pretense of niceties off of it) that all enlisted are lazy, cowardly, lying scum, and would refuse to even consider the testimony of any number of enlisted (no matter how senior or experienced), much less that of civilians, over a brother officer.
On “the plus,” your “Sarge” could be decorated and commended; on the downside, “Sarge” could have anything from a verbal reprimand and the “LT” quietly transferred out to a non-combat unit, to a full blown court martial for mutiny.
It really depends on the officers and senior NCOs at the next level or two “up” the food chain.
