>>715915951
Sans in Deltarune is almost certainly the same Sans that's in Undertale. For Sans, and probably Sans alone, Deltarune IS actually a prequel. His involvement in the story is to explain his character in Undertale.
He probably does know about the existence of multiple timelines already, but what that actually means depends on who Gaster is in Deltarune, since Gaster pretty clearly manages the saving and loading system here while in Undertale it's not under anyone's sole control.