Sans is to lazy to care about anyone besides his brother regardless
All of what you said is true though
In one OF the endings if you spare everyone but kill Papyrus, Sans will be pissed at you for killing him. But if you kill EVERYONE but SPARE Papyrus, Sans won't even care that you killed everyone else EXCEPT be happy that you spared his brother.
Sans would've known about the Genocide because the door to the ruins WOULD be open so he could see himself.
How else was Snowdin vaccuated?
Paps would've stayed to protect Snowdin from the Genocide Player in the first place.
During the time of the fight, Sans would've been with Asgore explaining everything to him about the Genocide Player(aka You, but as Frisk) and that's how Undyne would've known about you in the first place. Either that or the attention of the Snowdin folk brought her to that conclusion.
Although Sans is aware of the Resets, Papyrus IS to, because if you unlock the key to "Sans's room"(The laundry) Papyrus says the following "Is Sans pranking you through time and space again? God I hate it when he does that.."
That means Sans would've told Papyrus about the resets, or, what he knows about them in the first place.
True Resets are the only thing THEY AREN'T aware of because it erases both of their memories of you all together, and that INCLUDES everyone else.
The main point, Sans is to lazy to give a f**k
